CALIFORNIA 
AGRICULTURAL  EXTENSION  SERVICE 

CIRCULAR  20 

JUNE,   1928 


PEAR  BLIGHT  CONTROL  IN 
CALIFORNIA 


LEONARD  H.  DAY 


PUBLISHED  BY 

THE  COLLEGE  OF  AGRICULTURE 
UNIVERSITY  OF  CALIFORNIA 

Cooperative  Extension  work  in  Agriculture  and  Home  Economics,  College  of  Agriculture, 
University  of  California,  and  United  States  Department  of  Agriculture  cooperating.  Dis- 
tributed in  furtherance  of  the  Acts  of  Congress  of  May  8  and  June  30,  1914.  B.  H.  Crocheron, 
Director,  California  Agricultural  Extension  Service. 

UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1928 


CONTENTS 

PAGE 

Introduction 3 

Characteristics  of  the'disease 4 

Structure  and  physiology  of  the  pear  tree 8 

Seasonal  history  of  pear  blight  in  California 12 

How  blight  is  spread 14 

Reducing  the  number  of  insects  as  a  method  of  blight  "control 15 

Relation  of  cultural  methods  to  blight  control 16 

Soil  conditions 16 

Pruning 16 

Blight  control  by  use  of  resistant  roots,  trunks,  and 'scaffold  branches 18 

Surgical  methods  of  treating  blight  cankers 21 

Scarification 22 

Zinc  chloride  for  the  control  of  blight  cankers 25 

Directions  for  using  zinc  chloride 30 

Treatment  of  cases  not  cured  by  first  application 38 

Inspection  of  treated  cankers 38 

The  zinc-chloride  solution 40 

Antidote 40 

Directions  for  the  control  of  pear  blight  in  California 41 

1.  Use  of  blight-resistant  roots  and  bodies 41 

2.  Cultural  methods  of  blight  control 41 

3.  Insect  control 42 

4.  Calendar  of  blight-control  operations 42 

5.  Disposal  of  blight  cuttings 44 

6.  Treatment  of  badly  blighted  trees 44 

7.  The  blight  crew 47 

8.  Proprietary  remedies 47 

9.  Community  cooperation 48 

Formulas 48 

Formula  No.  1 — 43  per  cent  zinc  chloride 48 

Formula  No.  2 — 53  per  cent  zinc  chloride 49 

Formula  No.  3 — Mercury-glycerine  disinfectant 49 

Formula  No.  4 — Bordeaux-oil  paint 50 

Acknowledgments 50 


PEAR  BLIGHT  CONTROL  IN  CALIFORNIA 

LEONAED  H.  DAYi 


Pear  blight  {Bacillus  amylovoroics  Burrill)  first  appeared  in  the 
San  Joaquin  Valley  in  epidemic  form  in  the  spring^  of  1900  and  within 
two  years  had  practically  destroyed  the  pear  industry  in  that  section. 
The  disease  soon  spread  into  practically  all  the  pear-growing  districts 
of  the  state.  In  the  Sacramento  Valley  it  became  epidemic  in  the 
spring  of  1904.  An  educational  campaign  among  the  growers  was 
immediately  begun  by  the  Universitj^  of  California  Agricultural 
Experiment  Station,^  in  cooperation  with  the  United  States  Depart- 
ment of  Agriculture  and  the  office  of  the  State  Commissioner  of  Horti- 
culture, in  an  effort  to  acquaint  the  growers  with  the  nature  of  the 
disease  and  methods  of  control.  As  a  result  of  this  campaign  the  pear 
industry  of  the  Sacramento  Valley  was  saved.  In  the  coast  districts 
pear  blight  has  seldom  done  extensive  damage. 

Although  apparently  of  American  origin,  pear  blight  has  now 
spread  to  many  parts  of  the  world.  Throughout  the  states  of  the  East, 
South,  and  Middle  West,  the  disease  has  been  destructive  from  the 
beginning  of  the  pear  industry.  It  is  probable  that  if  it  were  not  for 
the  severity  of  the  disease  in  the  eastern  part  of  the  United  States  the 
Pacific  Coast  growers  would  not  be  able  to  compete  in  the  Barlett 
pear  markets  east  of  the  Rocky  Mountains. 

Pear  blight  has  often  been  called  'fire  blight'  because  infected 
twigs  and  fruit-spurs  wilt  and  turn  black  as  though  a  fire  had  swept 
through  the  orchard.  The  term  'fire  blight'  has  also  been  preferred 
-by  some  because  the  disease  has  host  plants  other  than  the  pear.  In 
California,  the  pear  tree  is  the  most  commonly  and  seriously  affected 
plant  and  the  disease  is,  therefore,  known  to  the  fruit  grower  as  '  pear 
blight. '  The  term  '  twig  blight, '  '  blossom  blight, '  '  root  blight, '  '  crown 
blight,'  'collar  blight,'  'body  blight,'  etc.,  have  been  used  to  designate 
infections  in  particular  parts  of  the  tree. 

Pear  blight  is  caused  by  a  species  of  bacteria  {Bacillus  amylovorous 
Burrill)  working  in  the  bark  of  the  tree.  It  is  spread  mainly  by 
insects  which  transfer  the  bacteria  from  infected  to  healthy  trees. 


1  Research  Assistant  in  Pomology. 

2  Smith,  Ralph  E.     Report  of  the  Plant  Pathologist  to  July  1,  1906.     Cali- 
fornia Agr.  Exp.  Sta.  Bui.  184:222.     1907. 


4  CALIFORNIA    AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

The  disease  is  usually  not  active  during  the  winter  months.  At 
blossoming  time  the  bacteria  in  the  diseased  portions  of  infected  trees 
become  active.  Gum,  laden  with  the  bacteria,  exudes  from  these  points 
of  infection  and  is  carried  into  the  blossoms  or  new  tender  twigs  by 
insects.  Beginning  thus  in  the  blossoms  or  twigs  in  the  early  spring, 
the  bacteria  travel  slowly  downward  in  the  bark  to  the  larger  branches, 
trunks,  and  roots,  producing  the  characteristic  gumming  cankers  in 
the  bark.  Insects  continue  to  spread  the  disease  throughout  the 
spring  and  summer. 

The  severity  of  the  infection  generally  depends  upon  the  vigor  of 
the  tree.  Vigorous  trees  with  much  succulent  growth  are  most 
susceptible.  In  these  the  disease  travels  rapidly  in  the  twigs  and 
bark,  often  advancing  several  inches  a  day.  On  this  account  pear 
blight  at  one  time  was  thought  to  be  caused  by  over-nourishment  due 
to  fertile  soils,  fertilization,  or  intensive  cultivation.  However,  Pro- 
fessor T.  J.  Burrill,  Botanist  of  the  University  of  Illinois,  about  fifty 
years  ago  proved  the  disease  to  be  infectious,  isolated  the  bacteria,  and 
grew  them  in  artificial  cultures. 

Control  of  the  disease  in  California  has  in  the  past  consisted  in 
cutting  off  the  diseased  fruit  spurs,  twigs,  and  larger  branches  as  fast 
as  they  appeared.  Pruning  tools  and  wounds  have  been  disinfected 
after  each  cut.  During  the  spring  and  early  summer  the  orchard 
has  been  gone  over  every  week  or  ten  days  in  an  effort  to  remove  as 
much  of  the  infection  as  possible  before  the  whole  branch  or  tree 
became  involved,  and  before  insects  spread  it  to  healthy  twigs. 

Efforts  at  complete  eradication  have  never  been  successful,  possibly 
because  the  disease  has  been  harbored  by  many  host  plants  and  carried 
over  from  season  to  season  in  small  hidden  infections  in  the  rough 
bark  or  even  in  the  bark  of  the  roots  of  the  pear  trees. 


[  CHARACTERISTICS   OF   THE    DISEASE 

Pear  blight  may  be  distinguished  from  other  diseases  by  the  sudden 
wilting  of  affected  blossoms  and  twigs  in  the  spring  and  early  summer 
(fig.  1).  Drops  of  a  whitish-colored  gum  usually  exude  from  such 
affected  parts,  and  upon  cutting  into  the  bark  of  freshly  wilted  twigs 
a  bright  red  color  is  evident.  The  leaves  turn  dark  but  do  not  drop. 
The  diseased  bark  soon  dies  and  turns  brown  inside  and  dark  at  the 
surface.  The  infection  progresses  largely  downward  in  the  bark, 
which  becomes  sappy  (or  red  and  sappy)  inside,  sometimes  for  six 
inches  or  a  foot  beyond  any  surface  discoloration. 


1928] 


PEAR    BLIGHT    CONTROL    IN    CALIFORNIA 


Fig.  1. — Blighted  pear  blossoms  and  twigs.     Note  the  dead  leaves  hanging  on 
tightly  and  the  drops  of  gum  on  the  twigs  and  fruit  stems. 


The  term  'canker'  is  applied  to  an  affected  area  on  a  large  branch 
or  trunk  of  the  tree.  The  disease  gains  entrance  to  these  older  por- 
tions usually  through  infected  fruit  spurs  or  smaller  branches.  The 
canker  may  spread  upward  or  downward  in  the  bark  from  the  point 
of  infection  (fig.  2,  A).  In  the  late  summer  and  fall  the  cankers  often 
do  not  exude  gum.  Dead  or  dormant  cankers  are  not  red  inside  but 
can  usually  be  distinguished  by  beads  of  gum  on  the  surface  or  by  the 
appearance  of  the  twigs  which  have  conducted  the  disease  into  the 


CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE 


[CiRC.  20 


branch.  Rains  often  wash  off  the  gum  but  the  dead  leaves  remaining 
attached  to  the  twig  are  a  characteristic  of  the  disease.  The  inner 
layer  of  bark  and  cambium  under  the  canker  may  or  may  not  be 
injured. 


Fig.  2. — A,  A  dormant  canker  of  pear  blight.  Note  the  twig  which  con- 
ducted the  disease  into  the  bark  of  the  large  branch.  The  leaves  are  dead  but 
hang  on  tenaciously. 

B,  A  papery-barked,  shallow  canker.  The  only  indication  of  blight  in  the 
branch  is  a  raising  or  blistering  of  the  epidermal  tissues — indicated  by  the 
arrows. 


The  disease  is  confined  to  the  outer  layers  of  bark  at  first,  but 
after  a  week  or  two  the  bacteria  penetrate  into  the  fibrous  inner  layers 
and  usually  kill  the  cambium — the  thin  layer  of  cells  between  the  bark 
and  the  wood  (fig.  3).  The  canker  may  be  confined  to  one  side  of  the 
branch  or  it  may  extend  all  the  way  around  it.  In  the  latter  case,  if 
the  cambium  is  killed,  the  parts  above  the  injury  slowly  wither  and  die. 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  7 

The  diease  may  run  its  course  and  stop  without  making  a  large 
canker,  or  it  may  continue  to  progress  downward  and  kill  the  whole 
tree.  In  some  cases  the  canker  stops  without  penetrating  to  the  cam- 
bium or  even  into  the  fibrous  inner  layer  of  bark.  In  case  the  cambium 
is  not  killed  it  soon  produces  a  new  layer  of  inner  bark.  The  edges 
of  a  dormant  canker  usually  crack  away  from  the  healthy  tissue,  in 
which  case  the  dead  shriveled  area  is  distinctly  set  apart  from  healthy 
bark  (fig.  2,  A).  The  disease  may  or  may  not  renew  activity  again 
later  in  the  season. 

The  term  'hold-over  canker'  is  used  to  designate  those  cankers 
which  remain  on  the  trees  over  winter.  Hold-over  cankers  may  be  on 
the  roots  or  in  the  rough  bark  of  the  trunk  or  scaffold  branches  of  old 
trees.  Small  cankers  in  rough  bark  are  often  overlooked  even  with  the 
most  careful  inspection.  This  makes  it  a  most  difficult  matter  to 
eradicate  the  disease  completely  from  an  orchard.  However,  the  fewer 
the  hold-over  cankers,  the  fewer  are  the  infections  the  following 
spring.  Almost  invariably  where  a  large  number  of  infections  occur 
on  a  small  group  of  trees  in  an  orchard  the  responsible  hold-over 
canker  may  be  found  nearby. 

The  terms  'dormant,'  'active,'  'slow,'  and  'half-dormant'  cankers 
have  been  used  to  describe  the  various  conditions  of  activity  of 
cankers.  In  active  cankers  the  disease  spreads  rapidly  in  the  bark 
and  the  bright  red,  juicy  condition  of  the  bark  may  extend  from 
several  inches  to  a  foot  or  more  beyond  the  blackened,  older  part  of 
the  canker.  It  is  in  this  sappy,  red  portion  that  the  bacteria  are  most 
abundant.  In  the  case  of  slow  or  half -dormant  cankers  the  progress 
is  not  rapid  and  the  red,  juicy  condition  may  be  found  only  an  inch 
or  two  beyond  the  blackened  portion.  With  dormant  cankers  there  is 
often  no  visible  sign  of  bacterial  activity  at  the  margins,  and  deep 
cankers  usually  have  both  the  bark  and  cambium  killed.  In  some 
orchards  and  in  whole  districts  in  some  seasons,  a  majority  of  the 
cankers  are  shallow  and  the  cambium  layer  is  not  injured,  and  many 
of  the  branches  recover  even  though  the  disease  extends  completely 
around  the  branch.  Cankers  are  sometimes  so  shallow  in  the  bark  of 
large  branches  that  they  can  be  detected  only  with  considerable  diffi- 
culty. They  may  consist  of  mere  blisters  or  there  may  be  swollen 
areas  lifting  the  paperlike  outer  bark  (fig.  2,  B).  In  some  seasons 
rain  or  insects  carry  the  disease  into  the  half -grown  fruits.  A  milky 
gum  exudes  and  the  fruit  gradually  becomes  blackened. 

At  first  the  bacteria  generally  invade  only  the  more  or  less  granular 
outer  layers  of  the  bark  (fig.  3),  where  they  move  in  a  jelly-like  mass 


8  CALIFORNIA   AGRICULTURAL   EXTENSION    SERVICE  [CiRC.  20 

between  the  living:  cells  at  a  relatively  sIoav  rate.  The  cells  of  the 
outer  bark  contain  stored  foods  and  from  this  the  bacteria  derive  their 
nourishment  and  multiply  rapidly,  and  finally  cause  so  much  pressure 
as  to  break  small  openings  through  the  outer  bark  layer  with  the 
resulting  jelly-like  ooze.  This  gum  may  also  ooze  out  through  the 
lenticels  or  so-called  breathing  pores  in  the  bark  (fig.  13) .  Occasionally 
the  bacteria  travel  up  or  down  the  branch  in  the  inner  bark  near  the 
cambium  layer,  usually  in  a  streak  rather  than  in  a  wide  band.  In 
the  outer  bark  the  infection  may  progress  upward  as  rapidly  as  down- 
ward but  it  progresses  much  more  slowly  sidewise  around  the  branch. 
The  disease  travels  in  the  bark  of  rapidly  growing  trees  more  readily 
than  it  does  in  older,  slow-growing  trees.  On  entering  an  older  branch 
from  a  vigorous  twig  the  infection  often  stops  completely.  In  gnarly 
fruit-spurs  and  old  twigs  it  usually  progresses  a  short  distance  and 
dies  out.  In  fact  a  majority  of  infections  die  without  progressing 
down  to  the  main  branch.  In  the  large  branches  many  of  the  cankers 
often  stop  development  (spontaneously  to  all  appearances)  and  the 
bacteria  may  never  resume  activity.  The  reason  for  this  behavior  is 
not  definitely  known.  In  succulent  twigs  the  disease  apparently  some- 
times travels  in  the  sap-wood,  especially  upwards. 

Pear-blight  bacteria  do  not  live  long  when  exposed  to  sunlight  and 
dry  conditions.  In  hot  dry  weather  the  disease  does  not  spread  as 
rapidly  as  during  warm,  moist  weather.  Diseased  branches  cut  off 
during  the  summer  months  in  California  soon  dry  up,  if  not  piled 
too  deeply  or  in  damp  situations,  and  are  not  apt  to  be  serious  sources 
of  new  infections.  During  damp  weather  in  spring,  fall,  and  winter, 
blight  cuttings  may  be  the  source  of  new  infections  if  not  burned. 

Since  the  first  infections  in  an  orchard  usually  take  place  in  the 
blossoms,  pear  blight  does  not  become  epidemic  in  young  orchards 
until  the  trees  reach  the  blossoming  age.  Also  the  disease  is  not  found 
in  nursery  stock  in  California  because  the  trees  are  sold  when  only  one 
year  old.  In  the  eastern  states  the  trees  are  held  two  years  in  the 
nursery  and  often  sufficient  bloom  appears  the  second  year  to  make 
infection  possible  if  older  susceptible  trees  are  nearby. 

STRUCTURE  AND  PHYSIOLOGY  OF  THE  PEAR  TREE 

In  order  that  the  pear  grower  may  more  fully  understand  the  art 
of  controlling  blight  and  treating  cankers  and  diseased  trees,  the 
following  short  account  is  given  of  the  structure  of  the  pear  tree, 
together  with  some  of  the  principles  of  plant  growth. 

The  foods  used  by  the  tree  are  manufactured  in  the  leaves,  by  the 
aid  of  energy  derived  from  sunlight,  from  the  water  and  minerals 


1928" 


PEAR    BLIGHT    CONTROL    IN    CALIFORNIA 


taken  up  from  the  soil,  and  from  the  carbon-dioxide  gas  taken  in  from 
the  air.  The  soil  minerals  cannot  be  used  directly  for  food  by  the 
roots  or  other  parts  of  the  tree. 

MhxY  Cj/fc/c 
Oarer  BarH 


Fig.  3. — Diagram  of  6-year-old  pear  branch.  A  short  block  split  leugtliwise 
through  the  pith.  The  bark  is  shown  thicker  than  natural.  The  medullary 
rays  are  strands  or  bundles  of  cells  extending  from  the  outer  bark  into  the 
wood — only  a  few  of  these  strands  are  shown.  The  new  wood  next  to  the  bark 
is  commonly  called  'sapwood, '  and  the  older  wood  toward  the  center,  'heart- 
wood.'  Possibly  pear  branches  as  young  as  this  have  no  true  heartwood.  Note 
particularly  the  thin  cambium  layer  between  the  wood  and  inner  layer  of  bark. 

Water  from  the  soil  moves  upward  in  the  sap  wood,  and  according 
to  widely  accepted  opinion  all  of  the  food  manufactured  in  the 
leaves  moves  downward  toward  the  roots  in  the  inner  layers  of  bark 
(fig.  3).  From  this  point  of  view  it  is  evident  that  the  roots  will  be 
starved  if  the  bark  becomes  girdled  by  blight  and  killed  to  the  wood. 


10 


CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 


Fig.  4. — Natural  shedding  of  bark  of  a  pear  tree.  The  upper  patches  on  the 
right-hand  scaffold  branch  were  moistened  before  photographing  to  increase  the 
contrast.  This  is  the  beginning  of  the  natural  scaly-barked  condition  which  is 
shown  at  the  base  of  the  middle  scaffold. 


For  this  reason  a  tree  girdled  to  the  wood  in  the  upper  part  of  the 
trunk  must  be  cut  off  as  soon  as  discovered,  if  a  new  tree  is  to  be 
grown  from  the  roots  or  from  the  lower  part  of  the  trunk,  for  if  the 
roots  become  starved  the  new  sprouts  will  not  be  vigorous. 

The  wood  and  bark  of  a  tree  is  made  up  of  countless  millions  of 
microscopic  cells.     These  cells  are  like  small  boxes,  for  they  have  an 


1928]  PEAR   BLIGHT    CONTROL    IN    CALIFORNIA  11 

outer  wall  and  an  inner  caAaty.  The  interior  (or  cavity)  of  living" 
cells  is  filled  with  a  jelly-like  living  matter  (protoplasm),  together 
with  water  and  food  materials.  The  walls  of  all  cells  have  minute 
openings  into  the  adjoining  cells.  Between  the  walls  of  the  cells  of 
the  outer  bark  are  many  spaces  (intercellular  spaces).  It  is  in  these 
intercellular  spaces  that  the  pear-blight  bacteria  live.  However,  they 
finally  dissolve  the  cell  walls  and  enter  the  interior  of  the  cells.  The 
sap  movement  from  cell  to  cell  in  the  bark  of  a  tree  is  relatively  slow. 
This  is  probably  one  of  the  reasons  why  blight  bacteria  are  not  carried 
rapidly  throughout  the  tree. 

The  cambium  is  the  very  thin,  slippery  layer  of  cells  between  the 
inner  bark  and  sap  wood.  By  division  the  cambium  cells  produce 
new  wood  cells  and  water-conducting  tubes,  which  are  added  to  the 
outside  of  the  wood  layers.  Thus  the  tree  grows  in  diameter.  The 
cambium  cells  also  produce,  outwardly,  new  food-conducting  tubes 
and  bark  cells  to  take  the  place  of  those  which  are  broken  apart  and 
weathered  away  as  the  tree  grows  in  diameter.  When  blight  kills 
the  inner  bark  the  cambium  can  soon  produce  new  bark  cells.  But 
if  the  cambium  cells  also  are  killed  then  the  branch  soon  dies  above  the 
injury  because  there  will  be  no  new  bark  cells  formed. 

The  pear  tree  begins  shedding  (sloughing)  its  outer  layers  of  bark 
when  about  six  or  seven  years  old.  Patches  of  the  outer  layers  of 
bark  die  (fig.  4),  become  dark  in  color  and  shrivel,  leaving  sunken 
areas  varying  from  a  fraction  of  an  inch  to  several  inches  wide.  These 
patches  are  usually  much  longer  than  wide.  These  dark,  sunken 
areas,  look  like  dormant  blight  cankers  and  are  especially  confus- 
ing to  the  operator  when  a  blight  canker  lies  beside  these  patches. 
Often  the  blight  canker  will  not  run  far  beneath  these  sloughing 
patches  because  the  layer  of  outer  bark,  which  the  bacteria  would 
normally  live  in  is  dead,  but  as  these  patches  become  older  and  new 
bark  is  produced  (by  the  live  cambium  cells  beneath),  the  blight 
readily  runs  beneath  them.  The  operator  can  usually  distinguish 
these  sloughing  patches  from  cankers  by  the  fact  that  there  is  always 
normal,  healthy  bark  beneath  and  healthy  bark  at  the  sides.  The 
presence  of  a  blight  canker  can  also  be  distinguished  by  the  presence 
of  gum  or  of  dead  leaves,  dead  blossoms,  or  a  small  dead  shoot  through 
which  the  bacteria  entered  the  bark.  The  natural  dark  patches  finally 
begin  to  crack  and  peel  off  and  then  we  have  the  scaly-barked  condi- 
tion shown  in  the  tree  in  figure  4  at  the  base  of  the  middle  scaffold 
branch. 


12  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 


SEASONAL    HISTORY   OF    BLIGHT    IN    CALIFORNIA 

Usually  only  a  few  of  the  first  blossoms  are  infected  in  California, 
but  just  enough  infections  are  scattered  about  to  make  a  severe  epi- 
demic in  the  orchard  if  conditions  are  favorable  during  the  latter  part 
of  the  regular  blossoming  period  or  the  scattering  second  bloom.  Late- 
blooming  varieties  are  apt  to  have  a  severe  epidemic  of  blossom  blight 
if  adjacent  earlier  blooming  varieties  have  had  a  few  blossoms  affected. 

The  disease  next  appears  in  the  new  succulent  growth  which  is  pro- 
duced immediately  after  the  blossoming  period.  Insects  carry  the 
disease  from  infected  blossoms  to  these  tender  growths  and  later  from 
one  tender  growth  to  another.  The  disease  then  runs  down  these  twigs 
and  forms  cankers  in  the  larger  branches  and  trunks.  If  the  infection 
is  in  a  new  shoot  (sucker)  from  near  the  surface  of  the  ground,  the 
disease  may  quickly  extend  into  the  roots. 

Not  all  of  the  conditions  which  are  necessary  to  produce  these 
spring  epidemics  are  definitely  known.  Apparently  weather  condi- 
tions must  be  favorable  either  to  the  bacteria  or  to  the  particular 
insect  carriers,  or  to  both.  In  some  years  but  few  of  the  hold-over 
cankers  revive  in  the  spring,  or  else  they  revive  too  late  to  infect  the 
earlier  blossoms.  In  some  seasons  there  is  a  long  blooming  period 
and  this  is  favorable  to  blight  epidemics,  if  other  conditions  are 
favorable.  Continued  rain  or  foggy  weather  after  the  disease  has 
made  a  good  start  is  favorable  to  the  spread  of  the  disease.  Not  only 
are  the  bacteria  washed  down  from  the  upper  parts  of  the  tree  but 
they  will  live  longer  in  the  open  when  the  air  is  damp  and  the  sun  is 
not  shining  brightly.  Rains  during  May  and  early  June  (especially 
if  the  weather  is  damp  for  a  day  or  two  following)  will  cause  infec- 
tions in  the  scaffold  branches,  crotches,  trunks,  and  root  crowns.  Such 
a  condition  occurred  in  the  Sacramento  Valley  region  during  the 
second  week  of  June  in  1927  and  was  followed  by  the  most  severe 
epidemic  of  body  blight  that  has  been  experienced  in  that  region  for 
many  years.  The  infections  became  noticeable  in  ten  days  to  two  weeks 
after  the  rains. 

As  a  usual  thing  most  of  the  spring  cankers  begin  to  go  dormant 
during  June  as  the  flush  of  spring  growth  slows  down,  the  wood 
hardens,  and  the  fruit  begins  to  grow  rapidly.  In  the  case  of  the 
Bartlett  and  other  early  varieties  many  of  the  dormant  cankers  revive 
and  begin  active  development  after  the  fruit  is  harvested.  After 
harvest  the  new  shoots  begin  to  grow  again  at  the  tips,  new  buds  push 


1928]  PEAR   BLIGHT    CONTROL    IN    CALIFORNIA  13 

out  about  the  root  crown  and  trunks  and  often  some  out-of -season 
blossoms  open.  In  some  years  these  new  fall  growths  and  out-of- 
season  blossoms  become  infected  by  blight.  At  that  time  of  year  the 
cankers  either  do  not  exude  gum  or  do  not  do  so  until  after  the  blight 
has  reached  the  cambium  layer,  and  for  this  reason  the  cankers  are 
often  not  discovered  until  after  the  aifected  branch  or  tree  is  killed. 
In  some  years  the  loss  of  trees  is  much  greater  in  late  summer  and  fall 
than  in  spring  because  of  the  difficulty  in  finding  cankers  during  the 
later  season. 

Cankers  which  cease  development  before  harvest  time  and  then  do 
not  revive  in  the  fall  seldom  if  ever  renew  activity  thereafter.  On 
the  other  hand,  many  of  the  fall  cankers  which  cease  development  on 
the  approach  of  winter  will  renew  activity  the  following  spring  if 
conditions  are  favorable.  In  some  years  only  1  or  2  per  cent  of  the 
fall  cankers  renew  activity  the  following  spring,  but  in  other  years 
as  high  as  20  or  25  per  cent  on  large  branches  may  revive. 

In  some  years  practically  all  fall  cankers  become  dormant  by 
winter  time,  but  in  other  years  some  of  them  continue  to  develop 
slowly  or  even  actively  through  January  and  then  may  continue  slowly 
at  the  advancing  margins  of  the  canker  until  the  warm  weather  of 
spring  again  induces  rapid  progress.  At  this  latter  date  the  move- 
ment of  sap  again  becomes  active  and  much  oozing  of  gum  occurs. 
Infections  in  smaller  twigs  which  become  dormant  in  the  fall  dry  up 
and  do  not  usually  awaken  the  next  spring. 

Besides  the  seasonal  irregularities  in  the  prevalence  of  blight  in  a 
given  district  there  are  also  yearly  irregularities.  The  disease  is 
seldom  epidemic  for  more  than  one  or  two  years  at  a  time.  For 
instance,  in  the  Sacramento  Valley  the  disease,  though  present  every 
year,  has  appeared  in  virulent  epidemics  in  cycles  of  about  five  to 
seven  years  since  its  first  appearance  in  1904.  After  a  year  or  two  of 
little  blight  the  disease  becomes  more  active  and  gradually  becomes 
virulent  after  two  or  three  years.  Then  it  gradually  becomes  less 
active,  usually  over  a  year  or  two,  though  sometimes  it  suddenly  fails 
to  reappear  in  abundance  the  season  following  a  severe  epidemic. 

The  reason  for  this  more  or  less  cyclic  or  periodic  behavior  is  not 
known.  It  may  be  due  to  some  natural  check  to  the  bacteria  them- 
selves, such  as  parasitism  or  a  weakening  of  the  bacteria,  or  the 
insects  particularly  active  in  the  spread  of  the  blight  may  occur  in 
cycles  (thus  corresponding  to  the  cycles  of  the  disease),  or  possibly 
the  trees  may  become  more  resistant. 


14  CALIFORNIA   AGRICULTURAL   EXTENSION    SERVICE  [CiRC.  20 

In  many  districts  pear  blight  has  not  appeared  at  as  regular 
intervals  as  in  the  Sacramento  Valley.  In  certain  coastal  districts 
the  disease  has  been  present  since  1905  but  has  appeared  only  in  a 
mild  form,  and  in  only  a  few  seasons  has  it  become  prevalent  enough 
to  attract  notice.  In  the  foothill  sections  of  the  Sacramento  Valley 
it  occasionally  becomes  epidemic  at  the  same  time  as  on  the  floor  of 
the  valley,  but  it  usually  disappears  suddenly.  The  same  has  been 
true  in  some  of  the  pear  districts  of  southern  California.  Apparently 
it  is  more  destructive  along  the  southern  coast  than  along  the  northern 
and  central  coasts  of  California. 

In  seasons  when  blight  is  not  epidemic  it  has  often  been  found 
difficult  to  make  artificial  infections.  For  instance,  in  an  experimental 
orchard  85  per  cent  of  the  inoculations  were  successful  and  resulted  in 
active  cankers  one  year;  whereas,  on  the  same  date  of  the  following 
year  and  in  the  same  trees,  only  6  per  cent  of  the  inoculations  were 
successful ;  and  the  cankers  resulting  therefrom  soon  ceased  develop- 
ment, and  all  natural  infections  in  the  orchard  behaved  in  the  same 
way. 

It  is  also  common  for  a  few  orchards  to  escape  severe  injury  one 
year  in  a  district  where  blight  is  in  epidemic  form,  and  then  the  next 
year  to  have  a  severe  infestation.  Or  the  disease  may  have  been  epi- 
demic in  an  orchard  one  year  but  not  the  second  year,  even  though  it 
is  epidemic  in  the  district  the  second  year.  The  reason  for  this 
behavior  is  not  definitely  known. 


HOW  BLIGHT  IS   SPREAD 

All  evidence  points  to  hold-over  cankers  as  the  source  from  which 
new  blight  infections  start  in  the  spring.  Ants  or  other  insects  carry 
on  their  mouth-parts  the  bacteria  from  the  gum  which  exudes  from 
the  reviving  cankers,  to  blossoms,  succulent  twigs,  fresh  wounds, 
growth  cracks,  and  soft  callus  tissue  of  healing  wounds.  Each  new 
point  of  infection  soon  becomes  a  source  of  bacteria  for  further 
infection. 

The  disease  may  be  carried  from  diseased  branches  to  healthy  ones 
by  pruning  shears,  hoes,  cultivators,  tractors,  harness  hames,  single- 
trees, tools  used  by  blight  operators,  etc.  Rains  may  also  carry  the 
disease  down  into  fresh  wounds  on  the  roots,  the  soft  callus  tissue  of 
healing  wounds,  growth  cracks  in  trunks,  and  crotches  and  scaly 
branches.  The  disease  sometimes  gains  entrance  through  the  lenticels 
on  branches  and  twigs   during  wet  weather  and   into  the   stomata 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  15 

(so-called  breathing  pores)  of  leaves.  Fresh  wind  brnises  are  also 
susceptible  to  infection  in  wet  weather.  A  very  common  point  of 
entry  of  the  bacteria  into  the  bark  is  shallow  wounds  in  the  forks  of 
branches  made  by  the  shoes  of  workmen  who  climb  into  the  trees 
instead  of  using  ladders.  There  is  also  evidence  that  birds,  such  as 
sap-suckers,  woodpeckers,  and  flickers  (yellow-hammers)  may  at  times 
spread  the  disease.  Flickers,  especially,  are  commonly  seen  pecking 
about  the  crowns  of  pear  trees  and  are  sometimes  responsible  for  new 
infections. 


REDUCING   THE    NUMBER   OF   INSECTS   AS   A   METHOD   OF    BLIGHT 

CONTROL 

Any  insect  capable  of  biting  into  or  penetrating  tender  twigs  and 
the  soft  callus  tissue  of  healing  wounds  is  a  possible  blight  carrier,  as 
well  as  the  many  kinds  of  flower- visiting  insects  which  may  carry  the 
bacteria  from  flower  to  flower,  or  from  oozing  cankers  to  flowers.  A 
number  of  growlers  have  reported  that  blight  infections  are  more 
numerous  in  their  orchards  near  hedges  and  other  mindbreaks — con- 
ditions which  probably  aid  insect  activity. 

The  flower-visiting  insects  especially  active  in  carrying  the  disease 
may  be  the  larger  kinds  (ants,  bees,  wasps,  blow-flies,  etc.)  in  search 
of  the  nectar  excreted  by  the  flowers,  or  they  may  be  insects,  large  or 
small,  which  bite  or  puncture  the  flower  parts.  Many  minute  species 
such  as  aphids  (plant  lice)  and  thrips  live  in  the  flowers  as  well  as 
in  the  opening  leaves  at  the  growing  points  of  tender  twigs.  Other 
insects  commonly  found  about  pear  trees  and  which  perhaps  often 
carry  the  disease  are  click  beetles,  diabrotica  (the  so-called  green  lady 
bugs  or  beetles) ,  tarnished  plant  bugs,  squash  bugs,  and  leaf  hoppers. 

Ants,  blow-flies,  plant  lice,  thrips,  and  leaflioppers  are  known  to 
carry  blight  from  disease  to  healthy  parts  of  trees  and  it  is  probable 
that  some  effective  method  of  dealing  with  these  insects  would  prevent 
many  infections. 

However,  it  may  be  that  there  are  so  many  kinds  of  insects  con- 
cerned that  not  a  great  deal  can  be  done  in  the  way  of  destroying 
them  to  prevent  the  spread  of  blight.  The  same  kind  of  insect  may 
not  be  responsible  each  year.  A  more  careful  investigation  of  this 
matter  might  yield  results.  A  number  of  experimenters  have  worked 
on  the  problem  but  the  results  to  date  are  not  conclusive.  Reimer^ 
succeeded  in  reducing  the  number  of  infections  in  the  blossoms  and 


3  Eeimer,   F.   C.     Pear  blight   control.     Blue  Anchor   Magazine    (California 
Fruit  Exchange,  Sacramento,  California),  Vol.  5,  No.  3,  p.  2,  March,  1928. 


16  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

twigs  by  the  use  of  Bordeaux  mixture  (3-6-50)  as  a  repellent,  sprayed 
on  the  trees  just  before  the  buds  opened  (in  the  pink  stage).  Ants 
and  thrips  in  particular  seemed  to  be  repelled  by  the  Bordeaux  spray. 

A  number  of  growers  in  Sacramento  County,  California,  have  tried 
this  method  at  the  suggestion  of  Mr.  A.  E.  Morrison,  Horticultural 
Commissioner  of  that  county.  A  few  of  these  experiments  yielded 
encouraging  results  but  most  of  them  did  not  reduce  the  number  of 
infections.  Possibly  the  spray  was  not  applied  at  the  right  time  for 
the  particular  insect  concerned  in  that  district. 

It  perhaps  will  well  repay  the  grower  to  use  nicotine  sulfate  (1  pint 
to  200  gallons  of  spray)  with  the  codling-moth  sprays  to  kill  plant  lice, 
thrips,  and  leafhoppers.  Summer  oils  also  may  be  added  to  the 
codling-moth  spray  for  these  insects^ — 2i/4  gallons  of  summer  oil  to  a 
200-gallon  tank.  Or  both  nicotine  sulfate  and  summer  oils  may  be 
used.  When  spraying  with  oils,  the  basic  rather  than  the  standard 
arsenate  of  lead  should  be  used.  Also,  in  using  summer  oil  be  careful 
to  remove  all  residues  of  sulfur  sprays  (lime-sulfur  solution,  sulfur 
pastes,  etc.)  sticking  to  the  inside  walls  and  lid  of  the  tank,  for  oil 
and  sulfur  combinations  are  injurious  to  leaves. 

The  relation  of  honey  bees  to  the  spread  of  blight  is  not  sufficiently 
understood  and  should  be  carefully  investigated.  Studies  by  Gossard 
and  Walton^  in  Ohio  seem  to  indicate  that  after  blossoms  have  been 
pollinated  for  over  three  days  they  are  not  apt  to  become  infected. 
They  suggest,  therefore,  that  there  should  be  many  bees  present  in 
orchards  during  the  first  days  of  the  blooming  period  in  order  to 
pollinate  the  blossoms  quickly  before  blight  has  much  chance  to 
spread.  Whether  this  holds  true  for  conditions  in  California  has  not 
been  determined.  If  it  does,  then  it  would  indicate  that  the  hives 
should  be  taken  away  from  the  orchard  before  the  late  blossoms  open. 

RELATION  OF  CULTURAL  METHODS  TO   BLIGHT  CONTROL 

Soil  Conditions. — It  has  already  been  stated  that  the  disease  is 
more  virulent  in  rapidly  growing  trees  than  in  trees  that  are  grow- 
ing more  slowly  and  thus  harden  their  new  growth  early  in  the 
summer.  In  fertile  soils  and  in  soils  supplied  with  adequate  moisture 
the  vegetative  vigor  of  the  tree  is  usually  greater  than  in  less  fertile 
soils.  In  the  eastern  states  it  has  been  found  very  helpful  to  keep  the 
orchard  in  sod  (planted  to  grass)  rather  than  giving  clean  cultivation. 
The  grass  apparently  uses  up  excess  nitrates  or  other  mineral  salts  and 
hence  prevents  succulent  growth. 

4  Gossard,  H.  A.,  and  E.  C.  Walton.  Dissemination  of  fir©  blight.  Ohio  Agr. 
Exp.  Sta.  Bui.  357:108,  123.     1922. 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  17 

Under  certain  condition.s  omitting  one  irrigation  may  be  sufficient 
to  bring  about  the  results  desired  in  making  use  of  sucli  methods. 
The  planting  of  summer  or  permanent  cover  crops,  as  has  been  prac- 
ticed to  some  extent  in  southern  Oregon,  in  order  to  keep  down 
excessive  vegetative  growth,  may  be  advisable  under  especially  favor- 
able conditions.  The  fact,  however,  that  it  takes  large  amounts  of 
irrigation  water  to  keep  both  cover  crop  and  trees  growing  must  not 
be  overlooked.  Good  judgment  must  be  exercised  so  that  neither  crop 
nor  trees  suffer. 

A  number  of  California  growers  have  had  considerable  success  in 
reducing  the  virulence  of  blight  by  using  less  irrigation  water  or  by 
growing  summer  cover  crops  of  weeds,  grass,  clover,  alfalfa,  and  the 
like,  to  take  up  the  excess  water  in  the  soil.  If  this  practice  is  fol- 
lowed it  may  be  necessary  to  hoe  around  the  trees  to  protect  them 
from  field  mice  which  may  live  in  the  cracks  of  the  dry  soil  about 
the  trees. 

Pruning. — A  moderately  light  method  of  pruning^  may  often  be 
used  to  advantage  to  discourage  excessive  succulent  growth.  This 
requires  some  experience  and  careful  observation  on  the  part  of  the 
grower,  for  what  may  work  in  one  condition  of  soil  (in  a  neighbor's 
orchard  for  instance)  may  not  work  in  his  own  orchard.  In  some  dis- 
tricts cutting  and  thinning  out  of  fruiting  twigs  seems  to  be  necessary 
to  secure  fruit  setting. 

The  cutting  out  of  water  sprouts  about  the  scaffold  branches  and 
of  suckers  from  the  root  crown  during  the  growing  season  has  often 
been  observed  to  induce  more  blight.  In  this  case  the  blight  may  be 
either  washed  down  from  above  by  rain  or  by  wet  fogs  or  carried  by 
insects  into  the  fresh  wounds  or  into  the  soft  callus  tissue  after  heal- 
ing begins.  The  cutting  off  of  these  sprouts  in  the  early  summer  also 
encourages  new  ones  to  start  as  soon  as  the  crop  is  harvested  in  the 
case  of  the  Bartlett  and  other  early  varieties.  These  pushing  buds 
are  readily  attacked  by  blight,  and  trunk  infections  are  thus  started 
at  a  time  of  year  when  the  cankers  run  deep  before  gumming  occurs — 
if  it  occurs  at  all. 

The  advisability  of  cutting  off  all  fruit  spurs  about  the  body  of 
the  tree  is  still  an  unsettled  question,  but  some  growers  in  the  Sacra- 
mento Valley  believe  that  cutting  these  off  only  encourages  water 
sprouts  to  take  their  places,  and  these  are  as  readily  infected  as  are 
the  fruit  spurs  and  blossoms.     It  is  perhaps  best  to  encourage  the 


5  Tufts,  W.  P.    Pruning  bearing  deciduous  fruit  trees.     California  Agr.  Exp. 
Sta.  Bui.  386:38.     1925. 


18  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

development  of  fruit  spurs,  especially  on  hanging  laterals  or  twigs 
about  the  lov^er  parts  of  the  scaffold  branches.  Blight  is  not  usually 
serious  in  the  regular  blossoms  of  the  Bartlett  and  off-season  blossoms 
do  not  often  occur  on  these  spurs  low  down  on  the  scaffold  branches. 
If  the  fruit  spurs  become  infected  they  are  readily  found  and  the 
parts  treated  to  stop  the  blight  in  ways  described  later  in  this 
publication. 

It  is  a  common  practice  to  convert  into  fruit  spurs  some  of  the 
smaller  water-shoots  located  on  the  scaffold  branches  on  the  inside  of 
the  tree.  This  is  done  by  leaving  the  smaller  shoots  uncut.  The  follow- 
ing summer  these  develop  fruit-spurs  and  become  fruiting  'hangers.' 
They  may  then  be  shortened,  if  desired,  by  pruning.  If  gnarly  fruit- 
ing twigs  of  this  type  become  infected  the  blight  travels  very  slowly 
in  them,  while  if  water  shoots  become  infected  the  disease  travels  very 
rapidly  down  to  the  scaffold  branch. 


BLIGHT  CONTROL   BY   USE   OF    RESISTANT   ROOTS,   TRUNKS,    AND 
SCAFFOLD    BRANCHES 

Most  varieties  of  the  pear,  apple,  and  quince  are  susceptible  to 
pear  blight,  although  some  varieties  and  species  of  these  are  prac- 
tically immune.  The  disease  has  been  found  also  on  other  species  of 
the  Rosaceae  or  rose  family.  In  California,  varieties  of  loquat,  coton- 
easter,  pyracantha,  hawthorne,  and  California  holly  (Toyon  or 
Christmas  berry)  are  commonly  infected.  The  following  plants  are 
also  susceptible  to  a  greater  or  less  extent :  rose,  plum,  cherry,  peach, 
service  berry,  mountain  ash,  red  raspberry,  blackberry,  and  straw- 
berry. Whether  some  of  these  and  other  ornamental  and  native  plants 
of  the  rose  family  assist  in  spreading  the  disease  in  years  of  epidemics 
has  never  been  determined. 

The  most  important  commercial  varieties  of  pears  are  susceptible 
to  blight,  but  the  degree  of  susceptibility  varies  considerably.  The 
compartive  resistance  among  the  commercial  varieties  varies  from  dis- 
trict to  district.  Apparently,  if  the  district  is  favorable  to  rapid  tree 
growth  of  a  given  variety  that  variety  will  be  more  susceptible  to  the 
disease  than  it  is  in  a  district  where  it  grows  more  slowly.  At  the 
University  Farm  at  Davis,  California,  the  following  order  of  increas- 
ing resistance  among  the  leading  commercial  varieties  has  been  deter- 
mined :  Eastern  Beurre,  Glou  Morceau,  Bartlett,  Bosc,  Comice,  Hardy, 
Anjou,  Clairgeau,  and  Winter  Nelis.  The  following  varieties  have 
also  been  severely  blighted  at  Davis:  Madeleine,  Fox,  Dana  Hovey, 
Flemish  Beauty,  Forelle,  and  Clapp  Favorite. 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  19 

As  a  usual  thing,  the  more  susceptible  the  variety  the  more  deeply 
the  blight  penetrates  into  the  bark  in  the  early  stages  of  the  disease. 
In  the  case  of  some  of  the  more  resistant  varieties  the  cankers  in  large 
branches  usually  run  their  course  and  die  out  without  killing  the 
cambium  and  innermost  layer  of  bark. 

There  is  some  difference  in  degree  of  susceptibility  of  blossoms  and 
young  twigs  even  in  the  same  variety.  Thus  with  the  Winter  Nelis, 
which  is  generally  considered  as  semiresistant  to  blight,  the  blossoms 
are  often  more  severely  attacked  than  is  the  case  with  the  Bartlett, 
which  is  regarded  as  very  susceptible. 

The  natural  resistance  of  certain  pears,  such  as  the  oriental  species 
(Pyriis  ussuriensis  and  Pyrus  calleryana) ,  has  led  to  the  use  of  seed- 
lings of  these  species  as  rootstocks  and  trunks  on  which  to  propagate 
the  Bartlett  and  other  commercial  varieties  which  contract  the  disease 
readily.  A  few  varieties  of  Pyrus  communis  (the  common  species  to 
which  all  California  commercial  varieties  belong,  usually  called  French 
pear)  are  resistant  to  the  disease,  but  the  seedlings  of  these  are  not 
usually  resistant.  The  following  are  some  of  the  resistant  French 
varieties:  Old  Home,  Surprise,  Estella,  Longworth,  Douglas.  These 
do  not  produce  fruit  of  high  commercial  quality.  Since  some  of  these 
resistant  French  varieties  make  better  unions  with  our  commercial 
varieties  than  do  the  oriental  varieties,  they  are  being  used  as  trunks 
and  scaffold  branches  on  the  tops  of  which  commercial  varieties  are 
grafted,  the  roots  in  these  cases  being  either  seedlings  of  blight- 
resistant  species  or  of  the  non-resistant  French  stock. 

Since  the  disease  usually  kills  the  tree  when  it  gets  into  the  trunk 
and  roots  of  non-resistant  pear  trees,  it  is  desirable  that  new  plantings 
of  non-resistant  commercial  varieties  be  top-worked  on  trunks  and 
scaffold  branches  of  blight-resistant  species  or  varieties. 

It  would  also  be  very  desirable  to  have  resistant  roots,  but  as  yet 
it  is  not  certain  that  a  suitable  one  has  been  developed.  Several 
combinations  are  being  tried  by  growers  but  it  requires  fifteen  or 
twenty  years  to  demonstrate  the  value  of  these  combinations ;  for, 
aside  from  blight  resistance,  there  may  be  some  undesirable  quality 
of  the  rootstocks  such  as  susceptibility  to  woolly  aphis,  oak  root 
fungus,  black-end  disease,  overgrow^th  of  root,  and  trunk  sensitiveness 
to  excess  water,  drought,  marly  subsoils,  or  alkali. 

A  combination  that  has  been  planted  by  a  number  of  growers  in 
the  past  two  or  three  years  consists  of  Pyrus  calleryana  rootstock.  Old 
Home  trunk  and  scaffold  branches,  and  on  top  of  this  the  Bartlett  or 
other  commercial  variety.    This  combination  seems  very  promising  in 


20  CALIFORNIA   AGRICULTURAL   EXTENSION    SERVICE  [CiRC.  20 

Oregon*^  and  may  prove  to  be  successful  under  some  conditions  in 
California  but  in  several  instances  there  is  evidence  that  the  Cal- 
leryana  will  not  do  as  well  as  the  French  root  in  soils  tending;  to  be 
wet  or  in  those  containing  small  amounts  of  alkali.  The  Calleryana  is 
resistant  to  woolly  aphis,  but  it  may  not  be  resistant  to  oak  root 
fungus,  since  the  other  oriental  species  that  have  been  used  are  not 
resistant  to  this  disease.  It  is  also  possible  that  black-end  disease  may 
develop  on  trees  grown  on  this  rootstock,  since  the  Bartlett  often  shows 
tliis  trouble  when  grown  on  other  oriental  stocks. 

A  few  growers  are  trying  a  combination  consisting  of  French 
seedling  roots,  the  Old  Home  variety  for  trunk  and  scaffold  branches, 
and  Bartlett  (or  other  commercial  variety)  for  the  top.  This  com- 
bination would  seem  to  be  safe,  because  in  this  case  all  three  varieties 
are  French  pears  and  they  all  unite  well  with  each  other.  The  French 
root  is  not  resistant  to  blight,  but  if  planted  with  the  graft  union 
several  inches  below  the  surface,  so  that  very  little  suckering  occurs, 
it  will  not  contract  the  disease  so  readil}^,  especially  with  a  non- 
blighting  body. 

The  French  root  was  the  standard  root  for  pear  trees  in  California 
until  recently  and  except  for  the  blight  has  been  very  successful. 
Reimer^  has  found  a  few  French  seedlings  which  are  resistant  to 
blight,  and  whose  seeds  produce  seedlings  a  large  percentage  of  which 
are  resistant  to  blight.  He  believes  it  possible  to  develop  extensive 
orchards  of  these  resistant  French  stocks  solely  for  the  production  of 
seeds  from  which  nurserymen  can  grow  blight-resistant  French  roots. 

The  use  of  resistant  roots  and  body  does  not  render  the  Bartlett 
top  immune  to  blight,  but  if  the  Bartlett  top  becomes  infected,  only 
the  top  is  lost.  A  new  Bartlett  graft  or  bud  can  then  be  placed  on 
the  Old  Home  scaffold,  and  the  tree  can  be  brought  back  into  bearing. 
This  combination  is  probably  best  secured  by  planting,  in  the  orchard, 
nursery  stock  consisting  of  one-year-old  Old  Home  on  French  seedling 
roots.  The  tree  is  then  grown  in  the  orchard  for  two  or  three  seasons 
until  it  has  considerable  size  and  has  been  trained  to  three  scaffold 
branches.  Buds  or  scions  of  the  commercial  variety  are  then  placed 
on  the  scaffold  branches  a  foot  or  two  out  from  the  trunk.  Some 
growers  prefer  to  bud  or  graft  the  commercial  variety  onto  the  second 
set  of  branches  instead  of  the  first  set,  thus  putting  in  five  to  seven 


G  Eeimer,  P.  C.  Blight  resistance  in  pears  and  characteristics  of  pear  species 
and  stocks.     Oreg-on  Agr.  Exp.  Sta.  Bui.  214:66-74.     1925. 

7  Eeimer,  F.  0.  Blight  resistance  in  pears  and  characteristics  of  pear  species 
and  stocks.     Oregon  Agr.  Exp.  Sta.  Bui.  214:66-74.     1925. 


1928]  PEAR   BLIGHT    CONTROL    IN    CALIFORNIA  21 

buds  per  tree.  Budding"  is  done  late  in  July  or  August  after  an  irri- 
gation. The  following  spring  just  before  growth  starts  the  branch  is 
cut  off  about  one-half  inch  above  the  bud.  The  Old  Home  makes  a 
very  good  body,  because  it  has  a  fine  spreading  habit  that  is  easily 
trained  into  a  good  form  of  tree.  Some  prefer  to  graft  rather  than 
bud  as  this  is  done  in  the  late  winter  when  work  is  more  slack  than  in 
the  summer  time.  Grafting  gives  a  more  uniform  tree  because  all  the 
branches  are  cut  back  to  the  same  height. 

About  sixteen  years  ago  the  so-called  Japanese  pear  (Pyrus  sero- 
tina)  was  generally  advocated  as  a  rootstock  for  commercial  pears  in 
California.  This  root  was  supposed  to  be  much  more  resistant  to 
blight  than  the  root  of  French  varieties.  It  had  long  been  used  with 
success  in  the  eastern  states  and  in  Oregon.  Approximately  50  per 
cent  of  the  pear  orchards  planted  in  this  state  since  that  time  have 
been  on  this  root.  It  is  now  known  that  the  Japanese  root  is  not 
immune  to  blight  and  that  it  does  well  only  under  certain  soil  con( 
tions.  It  cannot  stand  either  wet  soil  or  dry  soil  as  well  as  the  French 
root.  Also  the  fruit  of  Bartlett  and  some  other  varieties  on  the 
Japanese  root  is  subject  to  the  so-called  black-end  trouble  in  Cali- 
fornia and  is,  therefore,  not  a  desirable  rootstock  for  our  pears. 


SURGICAL  METHODS  OF  TREATING  CANKERS 

In  the  earlier  blight  w^ork  in  California  all  infected  branches  were 
cut  off.  If  the  disease  had  girdled  the  trunk  the  tree  was  either 
destroyed  or  sawed  off  well  below  the  infected  area,  especially  where 
the  canker  was  only  in  the  upper  part  of  the  trunk.  It  was  soon  dis- 
covered, however,  that  in  case  of  cankers  confined  to  one  side  of  a 
scaffold,  trnnk,  or  root-crown  the  canker  could  often  be  arrested  by 
shaving  off  the  bark  and  scraping  the  wood  for  a  long  distance  beyond 
the  evidence  of  disease  in  the  bark  (fig.  5)  and  applying  disinfectants. 
However,  even  the  most  careful  operators  often  failed  to  stop  a  large 
percentage  of  cankers  by  this  method. 

About  the  year  1916,  Mr.  Hayward  Reed^  in  his  orchard  near 
Sacramento  (following  the  suggestion  of  W.  P.  Womack,  of  Sacra- 
mento) began  trying  a  new  method  of  bark  surgery  which  is  now 
known  as  'scarification.'  He  had  noted  that  the  blight  works  in  the 
outer  layers  of  bark  first  and  had  found  that  by  shaving  off  this  outer 
layer  and  applying  disinfectants  that  the  disease  was  often  arrested 
and  the  inner  bark  and  cambium  layer  saved.     A  number  of  growers 


8  Weldon,  Geo.  P.     Pear  growing  in  California.     California  State  Comm.  of 
Ilort.,  Mo.  Bui.  7:364.     1918. 


22 


CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE 


[CiRC.  20 


then  adopted  this  method  and  used  for  a  disinfectant  Ileimer's 
formula,  which  consisted  of  cyanide  of  mercury  and  bichloride  of 
mercury,  500  parts  of  each  in  water.  This  solution  served  fairly  well 
in  years  when  blight  was  not  very  virulent  but  in  the  severe  outbreak 
beofinning  in  1920  it  failed  to  stop  a  large  percentage  of  the  cases 
treated. 


Fig.  5. — Old  method  of  canker  treatment,  used  when  the  canker  was  confined 
to  one  side  of  the  trunk  or  branch.  Now  advised  to  follow  zinc  chloride  treat- 
ment in  spots  where  cambium  is  killed  and  healing  of  edges  has  begun.  The 
dead  bark  should  be  scraped  off  and  the  exposed  wood  covered  with  Bordeaux- 
oil  paint. 


In  the  spring  of  1921  the  Division  of  Pomology  of  the  University 
of  California  undertook  a  study  of  methods  of  treating  blight  cankers. 
The  two  principal  lines  of  attack  were  the  use  of  various  disinfectants 
with  and  without  surgery. 

Scarification. — Various  depths  of  shaving  oif  (scarifying)  the 
outer  layers  of  bark  were  tried,  and  a  number  of  disinfectants  were 


1928] 


PEAR    BLIGHT    CONTROL    IN    CALIFORNIA 


23 


Fig.  G. — Bartlett  pear  tree  treated  by  the  scarilic-atioii  method.  The  blight 
had  spread  from  a  water-sprout,  just  below  the  crotches,  up  into  the  scaffolds 
and  half  way  down  the  trunk  and  all  the  way  around.  This  was  a  fresh  canker. 
The  white  lines  indicate  the  ends  of  the  scarified  area.  This  photograph  was 
taken  the  year  after  the  operation.  Note  the  vigorous  new  growth.  If  the 
operation  had  been  delayed  a  week  or  two  the  inner  bark  would  have  been 
injured  and  the  tree  left  in  a  weakened  condition. 


24 


CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 


Fig.  7. — Home-made  scarifier.  The  ruler  and  inch  squares  will  aid  the 
blacksmith  in  constructing  this  tool.  The  blade  is  made  from  quarter-inch, 
square  steel  stock.     The  point,  lower  edge  and  bend  are  sharpened. 

used.  One  of  the  most  successful  disinfectants  tried  consisted  of  500 
parts  each  of  cyanide  of  mercury  and  bichloride  of  mercury  in  a 
solvent  consisting  of  three  parts  glycerine  and  one  part  water  (see 
formula  No.  3,  p.  49).  The  outer  layer  of  bark  had  to  be  shaved 
practically  to  the  depth  to  which  the  disease  had  penetrated  inwardly. 
This  method  was  widely  adopted  by  the  growers  and  was  very  success- 
ful iu  the  hands  of  careful  operators,  especially  in  treating  new 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  25 

cankers,  but  in  the  case  of  older,  deep  cankers  the  percentage  of 
cured  cases  was  often  as  low  as  50  per  cent  or  less.  Root  cases  were 
especially  difficult  to  treat  by  scarification  because  it  was  difficult  to 
prevent  accidental  cuts  and  bruises,  made  while  digging-  around  the 
tree  from  becoming  infected.  This  method  was  also  too  laborious  and 
slow  to  be  very  practical  during  severe  epidemics.  Also  in  the  case  of 
deep  cankers  there  was  such  a  thin  layer  of  inner  bark  left  that  the 
branch  was  very  slow  in  recovering. 

The  scarification  method  may  have  its  place  under  certain  condi- 
tions, mentioned  later;  therefore,  the  following  directions  are  given 
regarding  the  method  of  performing  the  operation : 

In  older  parts  of  the  canker  the  dead  or  diseased  outer  bark  is 
shaved  inwardly  until  wide  streaks  of  healthy  tissue  appear  and 
nearly  all  of  the  discolored  streaks  are  removed.  At  the  active  margins 
of  the  canker  the  disease  is  usually  not  so  deep  but  nevertheless  the 
discolored  bark  should  be  practically  all  shaved  off.  Thick  bark  such 
as  occurs  in  crotches  and  the  collar  tissue  on  the  opposite  side  of  the 
branch  from  the  crotch,  old  bud-scars,  etc.,  should  be  pared  away  to 
the  fibrous  inner  layer  next  to  the  cambium.  Old,  dry  cankers  in 
which  the  cambium  is  entirely  killed  over  a  fairly  large  area  may  be 
treated  in  a  similar  way,  provided  the  cambium  is  not  injured  all  the 
way  around  the  branch.  After  disinfecting  the  scarification  wound 
and  allowing  the  disinfectant  to  dry,  all  exposed  wood  (where  no  live 
cambium  remains)  should  be  painted  with  Bordeaux-oil  paint  (see 
formula  No.  4,  p.  50)  to  prevent  the  entrance  of  wood-decaying  fungi. 
Figure  6  shows  a  scarified  Bartlett  pear  tree  in  which  there  was  a 
new  canker  at  the  base  of  the  scaffold  branches  and  upper  part  of  the 
trunk  that  had  passed  all  the  way  around  those  parts. 

A  heavy,  long-bladed  jackknife  is  very  good  for  performing  the 
scarification  work,  but  if  there  is  much  to  be  done  a  box  scraper  is 
better.  The  home-made  scarifier  shown  in  figure  7  does  the  work 
better  and  more  rapidly  than  either  of  the  above  tools. 


ZINC   CHLORIDE    FOR   THE    CONTROL   OF    PEAR-BLIGHT   CANKERS 

A  great  many  chemicals  have  been  tried  during  the  past  six  years 
in  an  attempt  to  discover  one  which,  without  ])revious  surgery,  will 
penetrate  the  bark  of  the  pear  tree,  kill  the  pear-blight  bacteria,  and 
yet  not  kill  the  branch  or  tree.  The  most  promising  of  the  materials 
found  was  a  solution  of  zinc  chloride  powder  in  denatured  alcohol, 
acidified  with  hydrochloric  acid. 


26  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

The  chief  difficulty  experienced  with  zinc  chloride,  as  well  as  the 
other  chemicals  for  the  treatment  of  blight  cankers,  was  in  finding  a 
concentration  (or  strength)  of  solution  that  could  be  used  on  branches 
of  all  sizes,  and  on  cankers  of  all  ages  without  'over-penetration'  and 
consequent  death  of  the  branch. 

It  was  found  that  the  absorption  of  chemicals  not  only  varied  with 
the  size  and  age  of  the  branch  but  also  that  trees  in  different  orchards 
acted  differently  in  this  respect.  For  instance,  in  trials  on  healthy 
branches,  in  some  orchards  a  53  per  cent  solution  of  zinc  chloride 
did  not  injure  the  cambium  layer  of  branches  as  small  as  11/4  inches 
in  diameter,  while  in  other  orchards  this  strength  penetrated  too  far 
into  the  bark  and  could  not  be  used  with  safety  on  branches  less  than 
21/2  or  3  inches  in  diameter.  The  older  part  of  the  canker  was  more 
readily  penetrated  than  either  the  advancing  margins  of  the  disease 
or  the  normal  bark  beyond  the  cankers  where  the  disease  was  not  evi- 
dent. Trials  over  a  period  of  several  years  were  made  in  an  effort  to 
overcome  these  difficulties  in  the  use  of  zinc  chloride.  Various  concen- 
trations of  the  zinc  chloride  in  the  alcoholic  solution  were  tried  exten- 
sively (especially  with  concentrations  of  33,  43,  53,  and  63  per  cent) 
with  the  hope  of  finding  a  solution  which  would  cure  the  largest  num- 
ber of  cases  with  the  least  amount  of  over-penetration  of  the  bark 
and  killing  of  the  branch.  It  was  feared  that  the  use  of  more  than  one 
strength  of  solution  might  make  the  process  too  complicated.  How- 
ever, during  the  blight  epidemic  of  1927  several  growers  had  their 
operators  use  two  solutions  of  zinc  chloride,  43  and  53  per  cent 
(formula  on  p.  48),  without  confusion,  so  that  we  now  believe  it  to  be 
entirely  feasible  to  use  two  concentrations. 

The  zinc  chloride  solution  has  been  used  extensively  by  one 
orchardist  over  a  period  of  three  years  and  by  two  others  for  two 
years.  During  the  past  year  (1927)  a  large  number  of  growers  in 
various  parts  of  California  used  the  material.  This  extensive  trial 
of  the  method  has  revealed  many  important  points  regarding  its  prac- 
tical application.  Besides  checking  up  the  work  of  many  growers  we 
have,  during  1927,  treated  about  five  hundred  cankers  in  orchards 
where  the  blight  was  severe.  The  results  of  these  trials  by  growers 
and  of  our  own  experiments  may  be  summed  up  as  follows : 

1.  Orchards  vary  widely  in  the  size  of  branches  that  may  safely  be 
treated  with  the  53  per  cent  zinc  chloride  solution.  In  some  orchards 
branches  as  small  as  1%  inches  in  diameter  were  successfully  treated, 
while  in  others  larger  branches  were  over-penetrated  and  killed. 
Branches  were  not  injured  below  the  treated  area  nor  below  a  point 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  27 

where  it  would  be  necessary  to  cut  off  the  branch  to  stop  the  blight  if 
canker  treatments  had  not  been  used.  Smooth-barked  branches  up 
to  1%  inches  in  diameter  were  successfully  treated  with  a  43  per  cent 
solution,  but  cankers  on  scaly-barked  branches  often  did  not  yield  to 
this  treatment.  This  weaker  concentration  often  saved  branches  as 
small  as  %  inch  in  diameter. 

2.  The  cambium  layer  is  often  killed  by  zinc  chloride  solution  if 
the  canker  is  over  ten  days  or  two  weeks  old,  especially  if  the  cankers 
are  much  darkened  before  treatment.  Older  cankers  on  trunks  and 
scaffold  branches  are  less  liable  to  be  over-penetrated  than  on  smaller 
branches.  Commonly  a  large  proportion  of  both  young  and  old 
cankers  are  very  shallow  and  in  that  case  they  are  not  so  readily 
over-penetrated. 

3.  During  the  spring  and  early  summer  the  zinc  chloride  solution 
seldom  fails  to  stop  the  blight.  Growers  reported  that  as  high  as 
98  per  cent  of  new  cankers  (or  shallow  old  ones)  treated  at  this  time 
of  year  were  permanently  arrested  and  the  branches  saved. 

4.  Trees  usually  absorb  zinc  chloride  solutions  more  freely  after 
the  new  growth  begins  to  harden.  This  hardening  occurs  about  the 
third  week  in  June  in  the  case  of  the  Bartlett  pear  in  the  interior 
districts  of  the  state.  However,  since  the  hardening  period  differs 
from  orchard  to  orchard  and  from  season  to  season,  it  is  not  best  to 
change  to  the  43  per  cent  solution  on  branches  larger  than  1%  inches 
in  diameter  without  first  trying  the  penetration  of  the  53  per  cent 
solution  on  healthy  branches  of  various  sizes,  as  directed  on  page  36. 
Less  gum  is  exuded  during  the  summer  and  fall  months,  hence  the 
cankers  are  not  so  readily  discovered.  The  percentage  of  cases 
stopped  by  zinc  chloride  is  less  in  the  fall  because  the  cankers  are  often 
not  discovered  until  they  are  comparatively  old  and  deep.  But  usually 
the  blight  was  stopped  in  over  80  per  cent  of  the  fall-treated  cases  and 
the  branches  saved  in  case  of  new  cankers  or  shallow  older  ones. 

During  the  late  fall  and  winter  and  up  to  the  time  the  trees  are  in 
full  leaf  in  the  spring,  the  bark  is  less  easily  penetrated  by  zinc- 
chloride  solutions.  However,  dormant  cankers  are  readily  cured  dur- 
ing this  period  because  zinc  chloride  penetrates  the  injured,  darkened 
bark  more  readily.  Active  cankers  are  not  as  readily  cured  during 
the  late  fall  and  winter  as  during  the  summer  because  the  bacteria 
under  the  healthy  bark  beyond  the  margins  of  the  discolored  area  are 
not  as  easily  reached  by  the  zinc  chloride  solution.  During  foggy 
weather  or  damp  nights  the  zinc  chloride  is  often  washed  off*  before  it 
has  a  chance  to  penetrate  into  bark  beyond  the  margins  of  the  canker. 


28  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

It  requires  from  one  to  three  days  for  a  sufficient  quantity  of  the  zinc 
chloride  to  penetrate  into  the  healthy  bark  to  be  effective.  These 
results  show  the  importance  of  cleaning  up  the  blight  before  late  fall 
and  winter. 

5.  Zinc  chloride  treatment  has  been  very  effective  in  cases  where  it 
is  impossible  to  tell  whether  or  not  the  disease  has  yet  passed  from  the 
infected  twig  or  spur  into  the  larger  branch.  It  is  with  cases  of  this 
kind  that  blight  cutters  take  chances,  hoping  to  save  the  larger  branch 
and  the  fruit  growing  thereon.  The  zinc  chloride  solution  apj^lierl  to 
the  larger  branch  (after  cutting  off  the  infected  twig  or  spur)  has 
seldom  failed  to  stop  the  infection.  Figure  12C  shows  such  case  before 
and  after  treatment. 

6.  Long  cankers  and  cases  in  which  two  or  more  cankers  are  so 
close  together  (figs.  lOB,  IIB,  and  12A  and  B)  that  a  continuous 
treatment  of  3%  to  5  feet  in  length  is  necessary,  are  often  not  success- 
fully treated.  In  these  cases  the  zinc  chloride  solution  creeps  down- 
ward and  gives  an  overdose  to  the  lower  parts  of  the  canker.  For  the 
same  reasons  roots  and  graft  unions  of  young  trees  (as  mentioned 
below)  are  often  over-penetrated  if  the  treatment  extends  from  the 
scaffold  crotches  down  to  those  parts. 

7.  The  graft  or  bud  union  of  young  trees  is  more  readily  injured 
by  zinc  chloride  than  are  other  parts  of  the  trunk  and  root-crown. 
Whether  the  blight  penetrates  more  deeply  at  that  point  has  not  been 
determined.  This  injury  is  especially  common  when  the  solution  is 
applied  from  the  root-crown  and  up  the  trunk  to,  or  above,  the 
crotches  (fig.  lOB).  In  these  long  cankers  the  zinc  chloride  solution 
creeps  downward  and  thus  increases  the  concentration  at  the  root- 
crown.  Treatments  at  the  root  crown  which  extend  only  a  short  dis- 
tance up  the  trunk  (fig.  IIA)  seldom  over-penetrate  the  graft  union 
unless  the  blight  canker  was  old  at  the  time  of  treatment. 

8.  Zinc  chloride  treatment  is  of  great  value  in  treating  cankers  on 
large  branches  shortly  before  the  fruit  ripens,  because,  even  if  the 
branch  is  eventually  killed  by  the  treatment,  the  fruit  usually  matures. 
If  the  diseased  branch  were  removed  as  soon  as  the  blight  appeared, 
as  formerly  was  the  common  method  of  attempting  to  control  blight, 
both  branch  and  fruit  were  lost.  With  the  zinc  chloride  treatment  the 
blight  is  stopped  and  the  fruit  is  usually  saved.  Even  with  cases  in 
which  the  cambium  is  killed  four  or  five  weeks  before  harvest  time  the 
branch  usually  does  not  wither  till  after  the  fruit  matures. 

9.  The  rains  in  early  June  of  1927  caused  many  cases  of  trunk  and 
root  blight  and  in  some  orchards  many  of  these  were  not  discovered 


1928] 


PEAR   BLIGHT    CONTROL    IN    CALIFORNIA 


29 


Fig.  8. — A,  A  new  pear  blight  canker  before  treatment.  The  arrows  indi- 
cate the  extent  of  the  canker.  The  infection  began  in  the  small  blackened 
shoot  near  the  center  of  the  canker.  The  bark  in  the  older  parts  of  the  canker 
is  beginning  to  turn  dark.  A  week  later  this  canker  would  have  been  too  old 
to  treat  without  danger  of  killing  the  cambium  layer. 

B,  The  same  as  A  after  stubbing  the  twigs  and  applying  zinc  chloride  solu- 
tion. The  solution  darkens  the  older  parts  of  the  canker  the  instant  it  is 
applied.  It  is  very  important  that  the  stubs  be  thoroughly  coated  with  the 
solution. 


until  they  were  too  old  to  be  treated  successfully.  Many  orchardists 
stopped  their  blight  work  during  picking  operations  without  attend- 
ing to  these  cases  and  as  a  result  many  trees  were  past  recovery  by 
the  end  of  the  harvest  season. 

Blight  cases  treated  a  few  hours  before  a  heavy  shower  of  rain  in 
June  were  successful  except  on  outside  branches  exposed  directly  to 
the  rain.    A  hard  rain,  how^ever,  would  undoubtedly  have  washed  off 


30  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

most  of  the  solution.  It  is  likely  that  in  the  fog  belt  along  the  coast, 
treatments  in  foggy  weather  or  when  the  nights  are  damp  will  not  be 
very  successful  in  stopping  blight  cankers.  Scarification  may  have 
to  be  used  instead  of  zinc  chloride  during  long-continued,  damp 
weather. 

10.  Compared  with  surgical  methods  of  treating  cankers  (new  or 
dormant)  or  with  complete  amputation  of  affected  branches,  the  zinc 
chloride  method  is  more  certain  in  arresting  blight,  except  possibly 
during  the  late  fall  and  winter.  Scarification  (page  22)  may  have  to 
be  substituted  for  the  zinc  chloride  treatment  on  active  cankers  in 
some  districts  during  the  late  fall  and  winter  because  of  the  slow  rate 
of  penetration  and  of  the  prevalence  of  rain  or  heavy  dews  at  night. 
The  zinc  chloride  treatment  has  not  been  tried  in  the  coast  districts. 
It  may  be  possible  that  even  in  the  spring  and  early  summer  the  fogs 
and  damp  nights  may  cause  poor  results  along  the  coast. 

The  scarification  method  will  usually  cure  older  and  deeper  cankers 
than  can  be  cured  with  the  zinc  chloride  treatment;  however,  with 
these  older  scarifed  cases  the  branch  does  not  recover  its  full  vigor 
so  readily  as  does  the  same  sort  of  case  when  successfully  treated  with 
zinc  chloride. 

11.  The  above  results  were  almost  entirely  on  the  Bartlett  variety. 
A  few  trees  of  Hardy  and  Bosc  were  treated  with  good  results.  It  is 
doubtful  whether  the  zinc  chloride  will  penetrate  sufficiently  in  the 
bark  of  such  varieties^  as  the  Madeleine  (in  which  the  blight  runs 
rather  deeply)  to  be  effective. 

Directions  for  Using  Zinc  Chloride. — The  solution  is  usually  car- 
ried in  a  fruit  jar  or  wide-mouthed  bottle  and  applied  to  the  cankers 
with  a  paint  brush  about  1%  inches  wide.  Round  and  oval  brushes 
are  especially  good  because  they  hold  a  large  amount  of  solution. 
Larger  brushes  may  be  used  for  large  scaffold  branches  and  trunks. 
If  an  open  jar  is  used,  only  enough  of  the  solution  to  last  about  an 
hour  should  be  taken  at  one  time  as  the  alcohol  evaporates  rapidly. 
A  heavy  wire  can  be  wrapped  about  the  neck  of  the  jar  to  serve  as  a 
handle  (fig.  9). 

In  applying  the  solution,  begin  about  16  inches  above  the  signs  of 
the  disease  in  the  bark.  Ijift  the  brush  carefully  from  the  solution,  so 
that  it  will  be  dripping-full  of  the  liquid,  and  then  paint  downward, 
allowing  the  solution  to  run  freely  over  the  surface  of  the  bark,  and 
finish  about  12  inches  below  the  last  signs  of  the  disease  at  the  lower 
end  of  the  canker.  In  this  way  the  solution  will  creep  into  all  the 
cracks  and  lenticels  (fig.  13).    Do  not  try  to  apply  less  material  to  a 


1928] 


PEAR    BLIGHT    CONTROL    IN    CALIFORNIA 


31 


Fig.  9. — Tools  for  tre.ating  pear-blight  cankers.  Note  the  folding  saw  which 
can  be  conveniently  carried  in  the  bucket.  The  drain  or  tiling  spade  is  veiy 
good  for  removing  soil  from  between  roots.  The  towel  is  kept  damp  and  used 
to  wipe  zinc  chloride  from  the  hands.  The  sponge  tied  to  the  shear  handle  is 
especially  useful  in  applying  disinfectant  to  cuts  when  clipping  twig  blight  in 
the  spring  and  early  summer. 


32 


CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 


Fig.  10. — A,  Trees  four  years  old  or  younger.  Use  only  43  per  cent  solution  on 
trees  four  years  old  or  younger.  However,  if  trees  are  exceptionally  large  for 
this  age  53  per  cent  solution  may  have  to  be  used  as  indicated  in  B. 

B,  Trees  five  to  ten  years  old  with  long  cankers.  Use  43  per  cent  solution 
on  smooth-barked  branches  up  to  1%  inches  in  diameter  and  on  the  lower  half 
of  long  cankers  on  trunks  or  roots  where  53  per  cent  is  used  on  the  upper  part 
of  the  canker.  Use  53  per  cent  on  all  parts  above  ground  over  1%  inches  in 
diameter  except  on  the  lower  half  of  long  cankers. 

canker  on  a  small  branch  than  on  a  large  one  as  this  practice  is  certain 
to  give  poor  results.  Use  the  weaker  solution  (43  per  cent)  in  treating- 
small  branches,  but  apply  it  freely.  The  solution  should  be  applied 
fully  around  the  branch  even  though  the  disease  does  not  extend  all 
the  v^ay  around — except  in  case  of  very  small  side-cankers  on  large 
branches  or  trunks. 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  33 

Avoid  spattering  the  zinc  chloride  solution  because  it  will  spot  the 
fruit  and  is  painful  to  the  eyes  and  more  or  less  harmful  to  clothing 
and  shoes. 

Very  few  inspection  cuts  should  be  made  in  the  bark  in  determin- 
ing the  extent  of  the  canker  because  the  solution  is  very  penetrating 
and  will  kill  a  large  spot  of  bark  to  the  wood  wherever  a  wound  has 
been  made. 

All  twigs  and  spurs  within  the  cankered  spot,  and  for  5  or  6  inches 
beyond,  must  be  cut  back  to  stubs  about  two  inches  long  (figs.  8,  12 A, 
and  12B).  If  the  twigs  are  cut  off  short,  the  solution  will  kill  a  large 
spot  of  the  cambium  layer  around  the  cut.  Be  careful  to  apply  the 
solution  well  beyond  the  last  stub,  for  the  pruning  shears  may  have 
infected  the  cuts  with  blight  even  though  the  disease  had  not  pro- 
gressed that  far.  All  except  the  cut  ends  of  the  stubs  must  be  covered 
with  the  solution.  Leaves  remaining  on  the  stubs  should  be  removed 
for  they  interfere  with  the  proper  application  of  the  solution.  It  is 
not  safe  to  leave  twigs  and  spurs  in  the  treated  area,  because  blight 
may  have  already  progressed  into  them,  even  though  they  may  appear 
healthy. 

In  case  the  disease  extends  down  a  twig  and  comes  within  8  inches 
of  the  larger  branch,  cut  off  the  twig  and  apply  zinc  chloride  to  the 
larger  branch  (fig.  12,  C). 

Scales  of  bark  should  not  be  removed  in  order  to  treat  scaly-barked 
trees  unless  these  are  already  loose  and  ready  to  fall  away  naturally. 
On  the  underside  of  horizontal  branches  it  is  sometimes  necessary  to 
remove  the  scales  in  order  to  apply  the  solution.  Scraping  or  cutting 
off  old  dead  bark  and  scales,  unless  very  carefully  done,  causes  wounds 
into  which  the  zinc  chloride  penetrates  and  kills  large  spots  of  the 
bark  and  cambium  layer.  Rough,  scaly  and  corky  bark  should  be 
very  thoroughly  saturated  with  the  solution. 

Figures  10,  11,  and  12  give  the  strength  of  solution  to  use  on 
cankers  on  the  roots,  root-crowns,  trunks,  and  branches  for  various 
sizes  of  trees  and  conditions  of  disease.  These  recommendations  are 
for  the  average  orchard.  There  are  orchards  in  which  the  zinc 
chloride  penetrates  more  freely  than  in  others  so  that  the  smallest 
size  of  branches  to  be  treated  with  the  stronger  solution  (53  per  cent) 
may  have  to  be  2^2  or  3  inches  instead  of  1%  inches.  But  this  can 
only  be  determined  by  the  experience  of  each  individual  grower  in  his 
own  orchard.  Young  rapidly  growing  trees  are  often  the  ones  which 
absorb  the  solutions  most  freely  and  the  branches  of  old  trees  less 
freely.    • 


34 


CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 


Fig.  11. — A,  Trees  five  to  ten  years  old  with  short  cankers.  Use  53  per  cent 
solution  on  short  cankers,  on  large  branches,  trunks  or  roots.  Use  43  per  cent 
on  small  branches  (up  to  1%  inches  in  diameter)  and  on  small  roots  (up  to  2l^ 
inches  in  diameter). 

B,  Trees  over  ten  years  old.  Use  53  per  cent  all  the  way  down  on  long 
cankers  aifecting  trunks,  and  root  crowns.  Use  43  per  cent  on  roots  up  to  six 
inches  in  diameter  when  these  are  at  the  lower  ends  of  long  cankers  being 
treated  with  53  per  cent.  Use  53  per  cent  on  a  scaly-barked  branch,  no  matter 
how  small  the  branch. 


1928] 


PEAR    BLIGHT    CONTROL   IN    CALIFORNIA 


35 


f 


3//f/)/ 


S5% 
■S5% 


f 


-3/^qhf 


Si% 

43% 


\f 


f 


-B//^/>/ 


■43%, 

A  B 

Fig.  12. — A,  Long  canker  (requiring  a  treatment  extending  S^/^  to  4  feet  in 
length)  on  a  branch  over  1%  inches  in  diameter.  If  the  branch  is  more  or  less 
vertical  use  53  per  cent  above  and  43  per  cent  below.  Note  that  all  twigs 
within  the  diseased  area  are  cut  to  stubs  about  1%  to  2  inches  long. 

B,  Two  cankers,  one  immediately  above  the  other  on  a  branch  over  1% 
inches  in  diameter.  If  the  branch  is  more  or  less  vertical  use  53  per  cent  on  the 
upper  canker  and  43  per  cent  on  the  lower  one. 

C,  Cases  in  which  there  is  doubt  as  to  whether  or  not  the  blight  has  yet 
passed  from  the  small  twig  into  the  main  branch.  Cut  the  twig  off  and  apply 
either  53  per  cent  or  43  per  cent  solution,  according  to  size  of  the  branch. 
Apply  it  a  distance  of  8  or  10  inches  above  and  below  the  twig  and  sidewise 
for  about  two  inches.  In  this  type  of  case  it  is  probably  best  to  cut  off  the 
branch  closely  rather  than  to  a  stub  as  shown  in  A  and  B.  A  large  spot  of 
bark  may  be  killed  where  the  stub  is  cut  closely,  but  this  will  soon  heal  over. 


36 


CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE 


[CiRC.  20 


Fig.  13. — Healthy  branch  treated  with  zinc  chloride  solution  and  the  bark 
shaved  off  three  days  later  to  show  the  reddish  spots  caused  by  the  penetration 
of  the  solution  into  the  lenticels  (so-called  'breathing-  pores').  Note  also 
other  lenticels  that  are  now  the  center  of  sunken  spots  to  the  right  of  tlie 
scarified  patch  and  normal  lenticels  above. 


Since  trees  differ  from  season  to  season  and  from  year  to  year  in 
the  readiness  with  which  they  absorb  zinc  chloride,  we  advise  that 
each  grower  apply  the  53  per  cent  solution  to  healthy  branches  of 
several  sizes  in  his  own  orchard  several  times  during-  the  blight 
season — preferably  May  1,  June  1,  and  July  1.  After  a  week  or  ten 
days  he  can  tell  how  small  a  branch  can  be  treated  without  injury. 


1928]  PEAR   BLIGHT    CONTROL    IN    CALIFORNIA  37 

The  lenticels  will  be  penetrated  but  the  inner  layer  of  bark  not 
severely  injured  (fig.  13).  A  narrow  strip  of  bark  is  often  killed  to 
the  wood  on  the  underside  of  the  branch,  but  this  is  of  no  conse- 
quence and  should  not  be  considered  in  making'  these  tests. 

When  in  doubt  as  to  which  solution  to  use  on  a  branch,  use  the 
stronger  (53  per  cent)  solution,  for  it  is  better  to  kill  the  branch  than 
not  to  stop  the  disease.  The  43  per  cent  solution  is  intended  for  use 
on  young  trees,  small  branches,  small  roots,  and  on  the  lower  parts  of 
long  cankers  on  large  branches,  trunks,  or  roots. 

It  will  aid  the  grower  in  studying  the  effect  of  zinc  chloride  solu- 
tions on  blight  if  he  will  use  some  system  of  marking  so  that  he  will 
know  what  percentage  of  zinc  chloride  solution  was  used,  the  date, 
whether  the  canker  was  new  or  old,  active  or  dormant.  One  grower 
used  large-headed  roofing  nails^ — one  nail  for  new  cankers  and  two 
nails  for  older  cankers.  Others  have  used  cloth  of  different  colors, 
paints,  shipping  tags,  etc.  It  is  important  to  know  which  cankers 
were  old  and  hence  likely  to  be  overpenetrated. 

In  order  to  prevent  possible  mistakes  in  using  the  two  solutions  in 
the  orchard  it  is  desirable  that  one  of  them  be  colored.  A  few  drops 
of  laundry  blueing  added  to  the  43  per  cent  solution  will  give  it  a 
blue  color  which  will  distinguish  it  from  the  53  per  cent  solution. 
This  color,  however,  is  not  sufficiently  intense  to  stain  the  bark. 
Growers  should  be  careful  about  adding  coloring  matters  intended  to 
stain  the  bark,  for  some  kinds  of  coloring  agents  might  reduce  the 
efficiency  of  the  solution  or  else  cause  injury  to  the  bark. 

Large  areas  in  which  the  bark  has  been  killed  to  the  wood,  on  one 
side  of  the  branch  only,  should  (after  healing  at  the  edges  has  begun) 
be  scraped  to  the  wood  (fig.  5)  and  the  wood  covered  with  the 
Bordeaux-oil  paint  (formula  No.  4,  p.  50)  to  prevent  wood  rots. 

In  cases  where  the  canker  was  too  old  when  treated  and  the  branch 
was  killed,  the  amputation  cut  should  not  be  made  very  far  below  the 
killed  area  because  new  sprouts  often  groAV  out  from  the  area  of 
healthy  bark  treated  with  zinc  chloride.  If  in  doubt  as  to  how  far  the 
branch  was  injured  cut  rather  high  or  else  leave  the  branch  until  the 
following  summer. 

It  is  highly  important  for  the  grower  to  understand  that  the  zinc 
chloride  solution  must  be  properly  applied,  and  that  the  treatment  has 
its  limitations.  Without  a  proper  appreciation  and  understanding  of 
these  matters,  the  grower  will  be  disappointed  with  results. 


38  CALIFORNIA   AGRICULTURAL   EXTENSION    SERVICE  [CiRC.  20 

Treatment  of  Cases  Not  Cured  hy  First  Application. — Except  on 
trunks  and  very  large  scaffold  branches  it  is  usually  not  possible  to 
successfully  re-treat  cankers  that  were  not  stopped  by  the  first  appli- 
cation. The  second  application  usually  kills  the  cambium  layer, 
especially  of  smaller  branches. 

It  is  sometimes  better  to  re-treat  by  the  scarification  process  (p.  22) 
than  with  zinc  chloride  solutions. 

If  a  case  has  to  be  re-treated  with  zinc  chloride,  be  careful  to  treat 
only  the  area  affected  and  only  as  much  as  necessary  of  the  older  parts 
of  the  canker  where  the  previous  treatment  had  already  penetrated 
deeply.  In  case  the  solution  runs  over  these  older  parts,  then  the 
most  of  it  should  be  sponged  up  with  a  large  damp  cloth. 

Usually  an  unsuccessful  treatment  will  be  too  far  advanced  before 
discovered  to  make  further  treatment  possible ;  amputation  is  then 
necessary. 

Inspection  of  Treated  Cankers. — Four  weeks  after  treatment  and 
two  or  three  times  during  the  summer  and  fall  all  treated  cankers 
should  be  inspected  in  order  to  locate  any  cases  in  which  the  blight  has 
not  been  stopped.  After  a  little  experience  the  blight  operator  can 
tell  whether  or  not  a  treatment  has  been  successful  without  cutting 
into  them.  Even  if  there  is  no  gumming  there  will  usually  be  areas 
with  a  wettish  or  oily  appearance  in  the  older  parts  of  the  treated 
canker  if  it  has  not  been  cured.  In  rough  bark  it  may  be  necessary 
to  gouge  into  these  areas. 

Again  just  before  the  blossoms  open  in  the  spring  all  of  the 
cankers  should  be  inspected  and  any  suspicious  spots  explored  with  a 
gouge  to  find  if  the  disease  is  awakening.  Figure  14  shows  the  best 
gouge  we  have  seen  for  this  work,  for  it  removes  the  chip  and  makes  a 
smooth  cut  which  is  easily  disinfected.  During  the  blossoming  period 
the  treated  areas  should  be  looked  over  again.  The  gouging  tools 
should  be  kept  disinfected  as  well  as  the  inspection  cuts  in  the  bark, 
because  the  disease  is  readily  carried  from  place  to  place  should  the 
inspector  accidentally  cut  into  a  tiny  live  infection  without  detecting 
it  (see  disinfectant  formula  No.  3,  p.  49. 

The  inspector  must  become  acquainted  with  the  appearance  of  the 
bark  (both  inside  and  outside)  in  treated  cases.  The  red  color  and 
oily  or  watery  appearance  inside  the  bark  often  does  not  disappear  for 
several  weeks  after  the  application  of  zinc  chloride  solution.  The 
zinc  chloride  itself  often  causes  a  reddish  color  in  the  bark,  but  this 
is  not  as  bright  a  red  as  that  caused  by  blight.  Zinc  chloride  also 
occasionally  causes  a  gummy  exudation  from  injured  lenticels  or  from 


1928] 


PEAR   BLIGHT    CONTROL   IN    CALIFORNIA 


39 


u 


Fig.  14. — Tools  for  gouging  bark  when  inspecting  for  pear-blight  cankers 
on  large  branches,  trunks,  and  roots.  The  U-gouge  is  made  from  a  half-round 
chisel,  as  in  the  middle  figure.  The  gouge  to  the  reader's  left  is  made  by- 
grinding  a  half-inch,  flat  chisel  to  a  point. 

the  older  parts  of  old  cankers  where  the  wood  may  be  injured.  A 
pinkish  cast  to  healing  tissue  and  to  regenerating  cambium,  and  red- 
dish streaks  in  the  bark  and  wood  are  rather  confusing  until  the 
operator  learns  to  distinguish  these  conditions  from  the  color  of  these 
tissues  in  the  case  of  active  blight.  Figure  13  shows  dead  and  reddish 
spots  in  healthy  bark  to  which  zinc  chloride  was  applied.     The  epi- 


40  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

dermal  layer  of  the  bark  was  shaved  off  several  days  after  the  appli- 
cation to  show  how  the  material  had  penetrated  the  lenticels  and  killed 
spots  in  the  outer  layers  of  the  bark. 

It  is  much  easier  to  find  treated  cankers  when  the  trees  are  wet 
with  dew  or  rain  because  then  the  cankers  (especially  on  smooth- 
barked  branches)  are  very  dark  compared  with  the  healthy  bark. 
However,  when  wet  it  is  not  as  easy  to  determine  whether  or  not  the 
disease  has  been  arrested  as  when  dry. 

The  Zinc  CJiloHde  Solution, — Directions  for  preparing  the  zinc 
chloride  solution  will  be  found  on  page  48.  This  solution  is  a  caustic 
and  penetrating  disinfectant.  However,  even  if  it  penetrates  to  the 
wood  in  case  of  an  old  canker  or  a  small  branch  it  does  no  injury  to 
the  branch  below  the  area  on  which  it  is  applied.  After  the  zinc 
chloride  has  become  neutralized  by  the  juices  of  the  bark  the  healing 
process  is  rapid  and  new  buds  may  start  where  the  solution  was 
applied.  Even  where  the  cambium  layer  has  been  discolored  by  the 
zinc  chloride,  there  are  often  enough  live  cells  uninjured  to  regenerate 
a  new  cambium  layer. 

When  the  zinc  chloride  penetrates  into  the  wood  it  may  move 
upward  for  some  distance  but  the  cambium  layer  over  this  darkened 
wood  usually  regenerates.  The  zinc  chloride  seldom  moves  downward 
in  the  wood.  In  the  bark  it  may  work  downward  or  upward  but  only 
for  a  short  distance  beyond  where  it  entered  these  tissues.  Except  on 
small  branches  and  with  very  long  cankers  there  is  seldom,  if  ever, 
more  damage  done  by  the  zinc  chloride  than  the  blight  had  done  at 
the  time  of  application. 

There  is  very  little  danger  to  persons  using  the  zinc  chloride  solu- 
tion. It  is  so  caustic  and  distasteful  that  it  is  not  likely  to  be  taken 
internally.  It  was  formerly  used  in  human  surgery  as  a  disinfectant 
and  for  use  in  cauterizing  or  'burning  out'  cancers  and  tumors.  It 
is  very  painful  and  irritating  to  the  eyes  and  a  large  quantity  acci- 
dentally spilled  into  the  eyes  might  cause  serious  injury.  The  material 
irritates  the  skin  of  some  persons  more  than  others.  It  will  burn  the 
skin  if  the  clothing  becomes  saturated  and  is  allowed  to  remain  in 
contact  with  the  skin  for  some  time.  However,  some  operators  have 
their  fingers  wet  with  the  solution  day  after  day  without  serious 
annoyance. 

Antidote. — In  case  of  accidental  swallowing  of  zinc  chloride  solu- 
tion use  whites  of  eggs  beaten  up  in  water,  or  olive  oil,  or  a  large 
amount  of  water  or  milk.  Do  not  use  emetics.  Send  for  a  doctor.  In 
case  any  large  quantity  gets  into  the  eyes  wash  them  out  with  olive  oil 


1928]  PEAR   BLIGHT    CONTROL    IN    CALIFORNIA  41 

or  whites  of  egg^s.  Washing  soda  in  water  will  neutralize  the  zinc 
chloride  on  the  hands  and  should  also  be  used  if  any  large  amount  is 
spilt  upon  the  clothing. 

Cotton  clothing  is  not  greatly  damaged  by  zinc  chloride,  but 
woolens,  shoes  and  leather  gloves  are.  Rubber  shoes  and  rubber  gloves 
are  not  harmed.  Brushes  will  last  longer  if  washed  after  using,  each 
evening,  in  a  solution  of  washing  soda. 


DIRECTIONS  FOR  THE  CONTROL  OF  PEAR  BLIGHT  IN  CALIFORNIA 

To  be  successful  in  the  control  of  pear  blight  it  is  necessary  that 
the  grower  have  a  thorough  understanding  of  the  disease  and  its 
relation  to  his  general  scheme  of  orchard  management.  The  recom- 
mendations which  follow  are  based  on  results  of  experiments  by  the 
author  and  on  observations  of  results  secured  by  successful  California 
growers. 

1.  Use  of  Blight -Resistant  Roots  and  Bodies. — All  pear  trees 
planted  in  districts  where  blight  is  apt  to  be  a  serious  factor  should 
have  blight-resistant  bodies.  It  would  be  very  desirable  also  to  have 
blight-resistant  roots,  but  at  the  present  writing  definite  advice  on  this 
point  cannot  be  given  (p.  18). 

2.  Cidtural  Methods  of  Blight  Control. — As  suggested  before  (pp. 
16-17)  the  grower  should  try  to  discourage  excessive  succulent  growth 
by  careful  regulation  of  irrigation  water  and  a  system  of  pruning  as 
light  as  is  consistent  with  the  production  of  regular  crops  of  fruit  of 
marketable  quality. 

Blight  operators  should  go  ahead  of  the  pruning  crew  because  if 
active  cases  are  present,  the  pruners  may  scatter  the  disease.  Water- 
shoots  about  the  body  of  the  trees  and  suckers  should  be  removed  only 
during  the  dormant  season.  Cutting  these  off  during  the  blight  season 
will  make  wounds  into  which  the  rain  may  carry  blight  from  the  infec- 
tions above.  Also  the  removal  of  these  sprouts  encourages  new  buds 
to  push  out  about  the  body  of  the  tree  immediately  after  harvest  (of 
Bartlett  and  other  early  varieties),  and  these  are  very  readily  inocu- 
lated with  blight  by  insects.  Cankers  on  these  new  growths  are  diffi- 
cult to  find,  especially  in  the  fall  when  there  is  no  gumming.  On 
account  of  blight  it  is  desirable,  in  training  young  trees,  to  leave  six 
or  seven  permanent  scaffold  branches  at  a  height  of  four  or  five  feet 
from  the  ground.  Then  if  blight  kills  one  or  more  of  these  scaffold 
branches  the  tree  may  be  readily  rebuilt  and  brought  back  to  its 
original  bearing  capacity. 


42  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

3.  Insect  Control. — We  are  not  in  a  position  at  this  writing  to  give 
definite  advice  regarding  control  of  insects  responsible  for  spreading 
blight    (p.   15). 

4.  Calendar  of  Blight-Control  Opey^ations. — Four  general  opera- 
tions in  the  control  of  blight  during  the  various  seasons  of  the  year 
are  recommended  as  f ollow^s : 

a.  The  Winter  Cleanup.  During  the  fall  and  v^inter,  search  for 
holdover  cankers.  If,  during  the  late  fall,  leaves  on  a  branch  or  tree 
color  up  early,  gouge  into  suspicious-looking  spots  in  the  bark  of  the 
branch  and  even  into  the  trunk  and  roots  belov7,  if  no  blight  is  dis- 
covered in  the  branch.  However,  other  diseases  or  conditions  than 
blight  may  cause  the  leaves  on  a  branch  or  tree  to  change  color  early 
in  the  fall.  If  many  trees  are  found  with  root  cankers,  the  root-crown 
of  all  trees  to  a  depth  of  10  or  12  inches  under  ground  should  be 
inspected  by  gouging.  This  should  also  be  done  after  any  year  in 
which  fall  blight  has  been  prevalent  in  the  orchard.  In  some  old 
orchards  many  roots  have  a  reddish  color  which  is  sometimes  mistaken 
for  blight.  In  these  conditions,  however,  there  is  not  the  sappy  or  oily 
appearance  of  blighted  tissue.  All  cankers  discovered  should  be 
treated  with  the  zinc-chloride  solution  if  the  roots  or  branches  affected 
are  not  too  badly  injured.  Treated  roots  and  crowns  should  be  left 
exposed  for  several  days  before  re-covering  with  soil  because  it  takes 
several  days  for  the  zinc  chloride  to  penetrate.  In  the  winter  time, 
during  foggy  weather,  active  cases  should  be  left  exposed  after  treat- 
ment for  several  weeks  to  make  sure  that  the  blight  has  been  killed. 
These  cases  should  be  marked  for  future  inspection. 

Just  before  blossoming  and  during  the  blossoming  period,  search 
for  holdover  cankers  and  inspect  once  more  all  cankers  treated  the 
season  before  to  detect  any  new  activity.  Oozing  cankers  during  the 
blossoming  season  are  responsible  for  infections  each  spring. 

&.  Spring  Operations.  During  the  latter  part  of  the  blooming 
period,  begin  to  inspect  the  trees  for  blighted  blossoms  and  twigs, 
especially  about  the  trunks  and  larger  branches.  Throughout  the 
spring  and  summer  try  to  cover  the  orchard  once  a  week  to  cut  out 
infected  twigs  and  to  treat  cankers,  in  order  to  keep  ahead  of  the 
insect  spreaders  of  the  disease.  The  greater  the  number  of  infections 
in  a  tree,  the  greater  the  number  of  new  infections  that  may  be  car- 
ried to  healthy  twigs.  Furthermore,  success  in  saving  the  larger 
branches  with  the  zinc  chloride  solutions  depends  upon  finding  the 
cankers  while  they  are  still  young. 


1928]  PEAR   BLIGHT    CONTROL    IN    CALIFORNIA  43 

In  removing'  twigs  and  branches,  cut  to  some  lateral  (if  possible) 
a  foot  or  more  below  the  last  signs  of  the  disease.  Examine  every 
piece  cut  off  to  make  sure  that  the  cut  was  made  far  enough  below 
the  disease.  If  the  disease  was  within  8  or  10  inches  of  the  main 
branch,  the  branch  should  be  treated  with  zinc  chloride  solution.  Do 
not  allow  the  cut  piece  to  fall  through  the  tree  because  it  may  scratch 
and  infect  other  parts. 

Trees  severely  killed  back  by  blight  throw  out  a  great  many  succu- 
lent branches  and  these  are  very  susceptible  to  blight.  On  this  account 
it  is  desirable  to  cure  as  many  diseased  branches  as  possible.  Yet  dur- 
ing a  severe  blight  epidemic  it  will  not  pay  to  waste  much  time  treat- 
ing small  branches  with  zinc  chloride  because  a  difference  of  two  or 
three  days  in  getting  over  the  orchards  may  make  the  difference 
between  saving  or  losing  many  large  branches. 

The  removal  of  late  and  out-of -season  blossoms  while  cutting  off 
blighted  twigs  assists  very  materially  in  lessening  the  number  of  infec- 
tions during  seasons  of  blight  epidemics.  Some  growers  begin  remov- 
ing these  late  and  out-of -season  blossoms  just  as  soon  as  they  discover 
that  the  blight  is  becoming  epidemic  in  their  orchards. 

Disinfect  all  cuts  and  wounds  and  keep  the  tools  and  shears 
smeared  with  the  mercury-glycerine  disinfectant  (formula  No.  3, 
p.  49).  A  water  solution  without  the  glycerine  is  also  effective,  but 
the  glycerine  disinfectant  remains  on  the  shears  longer  without  drying 
up  and  hence  is  more  efficient  in  the  hands  of  careless  operators. 

If  cuts  are  not  disinfected,  ants  and  other  insects  may  re-infect 
them  immediately,  especially  in  damp  weather  in  the  spring  and  early 
summer.  If  there  are  many  twigs  to  cut,  a  piece  of  cheap  sponge 
bound  tightly  to  the  wooden  part  of  the  shear  handles  (fig.  9)  and 
saturated  with  the  disinfectant  makes  a  very  quick  method  of  apply- 
ing the  disinfectant  to  the  cut  ends.  Breaking  off  diseased  twigs  and 
branches  instead  of  cutting  is  not  advisable  because  the  broken  ends 
are  not  easily  disinfected  and  so  insects  may  immediately  infect  the 
wounds  with  blight  bacteria. 

Do  not  use  zinc  chloride  for  disinfecting  cuts  for  it  is  too  penetrat- 
ing and  besides  if  used  too  freely  in  the  tops  of  the  trees  it  is  apt  to  be 
spattered  onto  the  fruit. 

c.  Blight  During  the  Harvest  Season.  Do  not  stop  the  blight  work 
during  the  harvest  season.  Keep  a  competent  man  inspecting  the 
trunks  and  main  branches  because  at  this  time  the  most  serious  infec- 
tions are  apt  to  be  overlooked.  In  some  seasons,  however,  the  summer 
and  fall  infections  occur  mostly  in  the  new  sprouts  and  off-season 


44  CALIFORNIA    AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

blossoms  which  appear  immediately  after  the  crop  is  off  (of  the  Bart- 
lett  variety).  At  this  time  of  year  the  most  serious  losses  of  trees 
often  occur  on  account  of  trunk  and  root-crown  infections  which  do 
not  ooze  out  gum  freely  in  the  fall. 

d.  Fall  Operations.  If  many  cases  of  blight  are  discovered  in 
trunks  and  root-crowns  at  any  time  during  the  summer  or  fall  all  the 
trees  should  be  inspected  immediately  by  gouging.  Failure  to  do  this 
might  cause  very  serious  losses.  Each  inspection  cut  as  well  as  the 
tools  must  be  disinfected  (formula  3,  p.  49).  The  U-gouge  (fig.  14) 
is  one  of  the  best  gouges  for  this  purpose. 

An  attempt  should  be  made  to  find  all  cankers  of  blight  before  late 
fall  and  winter  because  it  is  much  more  difficult  to  cure  active  cankers 
during  this  season  than  it  is  during  the  spring,  summer,  and  early 
fall  months. 

5.  Disposal  of  Blight  Cuttings. — During  hot  weather  the  infected 
twigs  on  the  ground  dry  rapidly  and  probably  are  not  a  serious  source 
of  infection.  During  the  spring  and  early  summer  it  is  well,  as  a 
precautionary  measure,  to  gather  and  burn  all  twigs  and  branches 
which  are  removed  on  account  of  blight.  These  cuttings  should  never 
be  thrown  in  large  piles  and  left  unburned  because  they  will  not  dry 
out  sufficiently  to  kill  the  bacteria  for  a  long  time.  Especially  before 
an  irrigation  the  fresh  clippings  should  be  gathered  because  it  is 
probably  possible  to  infect  the  root-crown  and  suckers  by  bacteria  in 
the  irrigation  water. 

6.  Treatment  foi'  Badly  Blighted  Trees. — The  fact  that  severe 
epidemics  of  blight  occur  only  one  or  two  years  at  a  time  makes  it 
possible  for  the  grower  to  repair  injured  trees  and  rebuild  the  orchard. 

In  case  a  tree  is  girdled  by  blight  and  the  cambium  is  killed  in  the 
upper  ]3art  of  the  trunl^  or  about  the  crotches,  the  trunk  should  be 
cut  off  below  the  canker  but  as  far  above  the  graft  union  as  is  safe. 
This  should  be  done  as  soon  as  it  is  discovered,  else  the  roots  will  be 
starved  and  not  be  able  to  support  a  vigorous  new  growth.  If  this 
occurs  in  early  summer  new  sprouts  from  the  trunk  may  grow  out 
immediately  after  sawing  off  the  trunk,  but  if  it  is  late  in  the  fall,  the 
new  sprouts  will  not  grow  out  until  the  following  spring.  Several 
sprouts  should  be  saved  about  three  inches  apart  in  order  that  the 
stump  may  heal  completely  across.  If  not  enough  sprouts  occur, 
grafts  should  be  put  in  the  following  winter  (February  or  March)  to 
fill  out.  The  cut  end  of  the  stump  should  be  covered  with  the 
Bordeaux-oil  paint  to  prevent  wood  rots.  In  figure  15  perhaps  too 
many  sprouts  were  left  as  scaffold  branches  for  the  future  tree. 


1928] 


PEAR    BLIGHT    CONTROL    IN    CALIFORNIA 


45 


Fjg.   15. — New  branches  growing  from   lower  part   of  the   trunk   after 
removing  the  top,  which  was  killed  l)y  blight. 


Fig.  16. — Bearing  pear  trees  which  had  been  injured  \>y  tdighl  ni\  igorated 
by  grafting  root  suckers  into  trunk  and  branches  above  the  injury. 


46 


CALIFORNIA   AGRICULTURAL   EXTENSION    SERVICE 


[CiRC.  20 


In  case  the  girdling  extends  to  the  graft  union  the  tree  can  be 
cut  off  below  the  graft  union  and  scions  of  the  Bartlett  or  desired 
variety  set  in  about  three  inches  apart  the  following  spring.  However, 
most  growers  prefer  to  take  out  trees  which  are  killed  to  the  graft 
union  and  replant  with  new  trees  the  following  winter.     It  is  usually 


Fig.  17. — A  three-year-old  inarch  with  three  inarched  seedlings. 


easy  to  grow  new  trees  in  a  young  orchard,  but  in  an  older  orchard  it 
requires  considerable  care  because  the  older  trees  rob  the  young  trees 
of  moisture  unless  irrigated  often. 

In  case  a  tree  is  injured  only  on  one  side  in  the  trunk  a  sprout 
from  below  can  be  used  as  a  bridge  graft  (fig.  16)  to  increase  the  sap 
flow  on  that  side.  If  the  injury  extends  down  to  the  root  a  seedling 
tree  can  be  planted  along  side  the  old  one  and  the  top  grafted  into  the 


1928]  PEAR   BLIGHT    CONTROL   IN    CALIFORNIA  47 

old  tree  above  the  wound.  This  is  called  inarching  (%.  17).  Perhaps 
the  best  way  to  ^raft  in  the  top  of  the  seedling  is  to  cut  out  a  slice  of 
bark  of  the  seedling  a  considerable  distance  below  the  tip  and  cut  out 
a  piece  of  bark  in  the  old  tree  (to  the  cambium  layer)  wide  enough 
to  receive  the  seedling.  Nail  the  seedling  down  with  a  small  brad  and 
wax  over  with  grafting  wax.  The  top  is  not  cut  off  until  after  the 
graft  has  united  well  to  the  old  tree  three  or  four  weeks  after  the 
operation.    Keep  sprouts  off  the  seedlings. 

In  case  no  sprouts  are  available  below,  a  bridge  graft  can  be  used 
to  increase  the  sap  flow.  The  scions  for  the  bridge  graft  should  be  of 
the  previous  season's  growth  and  about  the  diameter  of  a  lead  pencil. 
The  two  ends  of  the  scion  are  whittled  flat  on  one  side  for  about 
1%  inches.  Sections  of  the  bark  are  then  cut  out  at  both  ends  of  the 
girdle  to  fit  these  flattened  ends  of  the  scion.  When  in  place,  the 
cambium  at  the  edges  of  the  scion  will  be  in  contact  with  the  cambium 
layer  of  the  tree.  The  ends  are  then  fastened  down  with  brads  and 
the  union  covered  with  wax  to  exclude  air. 

7.  The  Blight  Crew. — Great  care  should  be  exercised  in  choosing 
men  for  blight  work.  Not  all  men  have  the  patience  and  habitual 
carefulness  to  become  good  blight  men.  Some  will  not  believe  that 
blight  is  infectious.  Such  men  do  more  harm  than  good  in  blight 
work. 

The  crew  should  be  under  the  supervision  of  a  competent  blight 
foreman.  Some  owners  of  large  orchards  divide  the  orchard  into 
sections  with  one  competent  operator  to  supervise  a  small  group  of 
blight  cutters  in  each  section.  The  zinc  chloride  treatment  of  cankers 
on  the  scaffold  branches,  trunks,  and  roots  should  be  performed  by  the 
supervisor  of  the  crew.  All  the  men  should  carry  the  zinc  chloride 
solution  for  use  in  the  upper  parts  of  the  tree  either  on  cankers  or  in 
cases  where  it  is  doubtful  whether  or  not  the  disease  has  yet  passed 
from  the  twig  into  the  main  branch  (fig.  12C).  The  men  should  be 
careful  in  the  tree  tops  not  to  spatter  the  solution  on  the  fruit. 

8.  Proprietary  Remedies. — The  pear  grower  is  cautioned  regarding 
the  adoption  of  new  kinds  of  blight  remedies.  There  is  perhaps  no 
other  orchard-tree  disease  that  has  been  so  widely  exploited  by  quack 
'tree-doctors'  as  has  pear  blight.  However,  some  of  these  remedies 
have  been  offered  in  good  faith  by  the  orginators.  This  disease  has 
so  many  peculiar  variations  in  behavior  that  it  is  very  easy  for 
orchardists  as  well  as  experimenters  to  be  misled  regarding  the 
efficiency  of  a  remedy.    A  remedy  should  be  tried  throughout  a  period 


48  CALIFORNIA   AGRICULTURAL    EXTENSION    SERVICE  [CiRC.  20 

of  at  least  two  years  and  with  virulent  types  of  cankers  before  passing 
final  judgment  upon  it.  It  should  be  kept  in  mind  that  a  remedy 
which  works  in  one  district  may  not  work  in  another. 

In  California,  manufacturers  and  dealers  in  remedies  for  plant 
diseases  and  insect  pests  must  be  under  license  by  the  State  Depart- 
ment of  Agriculture.  A  license  is  not  issued  unless  the  remedy  has 
considerable  promise,  and  if  it  proves  worthless  the  license  will  be 
revoked  or  at  least  will  not  be  renewed. 

9.  Community  Cooperation. — The  matter  of  community  coopera- 
tion in  blight  control  is  very  important  but  pear  growers  have  experi- 
enced much  difficulty  in  getting  together  on  a  working  program 
aifecting  a  whole  community  or  district. 

The  County  Horticultural  Commissioners  can  do  very  little  in  the 
enforcement  of  horticultural  laws  unless  the  growers  are  whole- 
heartedly behind  the  movement.  There  are  in  most  districts  many 
absentee  owners,  and  small  non-commercial  plantings.  There  are  also 
ornamentals  in  home  yards  and  native  rosaceous  plants  which  may 
harbor  the  disease  over  winter  or  assist  in  its  spread  at  blossoming 
time.  The  expense  of  a  campaign  to  take  care  of  all  sources  of  infec- 
tion would  be  greater  than  some  counties  would  care  to  provide  for 
without  the  hearty  cooperation  of  most  of  the  growers  and  farm.ers 
in  the  district  affected. 

FORMULAS 

Formula  No.  1 — 43  Per  Cent  Zinc  Chloride. — Both  strengths  are 
prepared  by  dissolving  dry  zinc  chloride  powder  in  the  following 
solvent  in  an  enameled  kettle : 

Solvent : 

1  gallon  denatured  alcohol 
1  pint  water 

3  ounces  concentrated  hydrochloric  acid. 
Dissolve  6  pounds  of  zinc  chloride  in  the  above  quantity  (9  pints) 
of  solvent.  In  dissolving,  the  solution  becomes  very  hot.  Stir 
thoroughly,  while  hot,  and  crush  all  hard  lumps  with  a  wooden  paddle, 
until  thoroughly  dissolved.  Avoid  as  much  as  possible  inhaling  fumes, 
for  the  chemicals  used  in  denaturing  the  alcohol  are  injurious.  Cool 
as  rapidly  as  possible,  covering  with  a  lid  to  prevent  evaporation. 
Add  a  few  drop  of  laundry  blueing  to  give  it  a  bluish  color. 

Keep  in  large  bottles,  well  corked.  If  much  of  the  alcohol  evapo- 
rates the  strength  of  the  solution  becomes  greater.  Precipitates  (sedi- 
ments) consist  of  impurities.     It  is  not  necessary  to  stir  the  solution 


1928]  PEAR    BLIGHT    CONTROL   IN    CALIFORNIA  49 

before  using  because  the  sediments  are  not  useful.  Keep  the  dry 
zinc  chloride  powder  well  sealed  for  it  absorbs  moisture  very 
rapidly.  It  would  be  impossible  to  weigh  out  the  proper  amount  of 
moisture-laden  zinc  chloride  because  it  may  absorb  more  than  its  own 
weight  of  water.  Small  lots  of  dry  zinc  chloride  may  be  transferred 
to  Mason  jars  with  rubber  gaskets  to  keep  out  moist  air.  It  is  not 
necessary  to  use  chemically  pure  materials  for  making  up  zinc-chloride 
solution.  The  43  per  cent  solution  can  also  be  prepared  by  diluting 
1  gallon  of  the  53  per  cent  solution  (formula  No.  2)  with  three  pints 
of  the  solvent. 

Formula  No.  2 — 53  Per  Cent  Zinc  Chloride. — Dissolve  9  pounds 
of  dry  zinc  chloride  in  9  pints  of  the  solvent.  The  53  per  cent 
solution  is  prepared  in  the  same  way  as  the  43  per  cent  solution, 
except  that  blueing  is  not  added.  The  same  precautions  should  be 
exercised  in  preparing  and  storing  it. 

Formula  No.  3. — Mercury -Glycerine  Disinfectant. — 

1  quart  distilled  water  (or  rain  water) 
3  quarts  commercial  glycerine. 
16  tablets  cyanide  of  mercury  (or  i/4  oz.  of  the  crystals) 
16  tablets  bichloride  of  mercury  (or  %  oz.  of  the  crystals). 

Add  the  w^ater  to  the  glycerine  in  a  clean  enameled  kettle.  Heat 
to  165°  or  175°  Fahrenheit.  Remove  from  the  stove.  Dissolve  the 
cyanide  of  mercury  in  this  by  stirring  rapidly  with  a  clean  dry  stick, 
glass  rod,  or  enameled  spoon.  Next  add  the  bichloride  of  mercury  and 
stir  until  dissolved.  The  tablets  are  the  71/2  grain,  or  half -gram  size, 
known  also  as  'large'  tablets.  The  powder  or  crystal  form  of  the 
pure  compounds  is  usually  much  cheaper  than  the  tablets.  Glycerine 
should  be  purchased  in  1-gallon  or  5-gallon  lots  and  should  be  the 
commercial  and  not  medicinal  grade  for  the  latter  is  more  expensive. 
This  solution  will  keep  for  a  long  time  in  bottles,  well  stoppered. 

Mercury  compounds  are  very  poisonous.  Keep  the  tablets  under 
lock  and  key,  for  they  are  very  attractive  to  children.  In  case  of 
accidental  poisoning  the  following  is  the  simplest  of  the  official  treat- 
ments :  Administer  white  of  eggs  and  then  induce  vomiting  by  use  of 
mustard  water  (2  to  4  teaspoonfuls  of  mustard  thoroughly  stirred  into 
a  cupful  of  warm  water) .  Send  for  a  physician  or  rush  the  patient  to 
the  nearest  hospital. 

This  disinfectant  solution  is  economically  and  efficiently  applied 
with  a  i^aint  brush  (about  II/2  inches  wide).  Swabs  waste  too  much 
material.  When  doing  frequent  inspection  work  with  a  jackknife  the 
pocket  sponge-bottle  (fig.  14)  is  very  convenient.     The  sponge  is  kept 


50  CALIFORNIA   AGRICULTURAL   EXTENSION    SERVICE  [CiRC.  20 

saturated  with  the  disinfectant  and  carried  in  the  pocket  bottle  with 
the  lid  off,  thus  being"  always  in  readiness  for  use.  Operators  become 
careless  about  disinfecting  if  the  disinfectant  is  not  in  handy  form. 

Formula  No.  4. — Bordeaux-Oil  Paint. — The  Bordeaux-oil  paint  is 
made  by  stirring  raw  linseed  oil  into  any  one  of  the  Bordeaux  powders 
found  on  the  market — not  the  'two-package'  Bordeaux.  It  has  been 
used  by  a  number  of  fruit  growers  in  Oregon  for  five  or  six  years  and 
is  recommended  by  the  Oregon  Experiment  Station^  as  more  satisfac- 
tory than  lead  paints  or  the  Bordeaux  paste  made  with  water.  It 
makes  a  covering  which  lasts  for  at  least  four  years  and  prevents 
infections  of  wood-rotting  fungi. 

The  paint  can  be  made  up  in  a  metal  container  or  in  jars.  Add  the 
raw  linseed  oil  slowly,  while  stirring,  until  a  thick  smooth  paint  is 
formed.  Pouring  back  and  forth  from  one  container  to  another  will 
help  even  the  mixture.  It  is  desirable  that  the  paint  be  a  little  thick 
when  first  prepared  because  it  becomes  thinner  on  standing.  Apply 
with  a  paint  brush.  Wash  the  brush  with  coal  oil,  benzene,  or  gasoline 
before  setting  it  away  to  dry. 


ACKNOWLEDGMENTS 

The  writer  wishes  to  express  his  appreciation  to  the  following  who 
have  aided  in  this  work  at  one  time  or  another:  Dr.  Warren  P.  Tufts, 
upon  whose  suggestions  the  experiments  in  treatment  of  pear-blight 
cankers  were  begun,  for  valuable  suggestions  and  for  assistance  in 
preparing  the  manuscript  and  illustrations;  to  the  members  of  the 
Pomology  Division  at  Davis  and  to  Dr.  C.  S.  Bisson  and  Dr.  W.  W. 
Robbins  for  valuable  suggestions;  to  the  following  County  Farm 
Advisors  who  have  been  very  helpful  in  studying  the  results  secured 
by  pear  growers  in  the  use  of  the  zinc-chloride  treatments:  Roy  D. 
McCallum,  B.  J.  Jones,  and  W.  D.  Norton;  to  the  following  pear 
growers  for  valuable  suggestions  and  trials  in  their  orchards :  Howard 
Reed,  Hay  ward  Reed,  Geo.  F.  Otis,  Leland  L.  Stoll,  Mercer  Runyan, 
David  Elliott,  and  Frank  A.  Cleland.  Thanks  are  especially  due  to 
the  Earl  Fruit  Company  and  to  the  California  Pear  Growers  Associa- 
tion for  renting,  for  the  College  of  Agriculture,  a  pear  orchard  near 
Marysville  in  which  much  of  the  earlier  experimental  work  was  done. 


9  Zeller,  S.  M.    Cankers  of  apple  and  pear  in  Oregon  and  their  control.     Oregon 
Agr.  Exp.  Sta.  Cir.  73:1-29.     1926. 


STATION  PUBLICATIONS  AVAILABLE  FOR  FREE  DISTRIBUTION 


BULLETINS 


No.  No. 

253.  Irrigation  and   Soil  Conditions  in  the  3  89. 

Sierra   Nevada   Foothills,   California.  390. 

262.  Citrus   Diseases   of   Florida   and   Cuba 

Compared  with   those   of   California.  391. 

263.  Size  Grades  for  Ripe  Olives. 

268.   Growing  and  Grafting  Olive  Seedlings.  392. 

277.  Sudan  Grass.  393. 

278.  Grain   Sorghums.  394. 

279.  Irrigation   of   Rice  in   California. 
283.  The  Olive  Insects  of  California. 

304.  A  Study  of  the  Effects  of  Freezes  on  395. 

Citrus  in  California. 

310.   Plum  Pollination.  396. 

313.   Pruning      Young      Deciduous      Fruit 

Trees.  397. 

324.   Storage  of  Perishable  Fruits  at  Freez- 
ing Temperatures.  398. 

328.   Prune    Growing  in    California.  400. 

331.   Phylloxera-resistant  Stocks.  402. 

335.   Cocoanut   Meal    as    a    Feed    for    Dairy  404. 

Cows   and   Other   Livestock.  405. 

340.   Control     of     the     Pocket     Gopher     in  406. 

California.  407. 

343.  Cheese   Pests  and  Their   Control. 

344.  Cold    Storage    as    an   Aid   to   the   Mar- 

.     keting  of  Plums,  a  Progress  Report.  408. 

347.  The  Control  of  Red  Spiders  in  Decid-  409. 

uous  Orchards. 

348.  Pruning  Young  Olive  Trees. 

349.  A    Study    of    Sidedraft    and    Tractor 

Hitches.  410. 

350.  Agriculture     in      Cut-Over      Redwood 

Lands. 

353.  Bovine    Infectious    Abortion,    and    As-  411. 

sociated  Diseases  of  Cattle  and  New- 
born  Calves.  ^  412. 

354.  Results  of  Rice  Experiments  in  1922. 

357.  A    Self-Mixing    Dusting    Machine    for 

Applying  Dry  Insecticides  and  Fun-  414. 

gicides. 

358.  Black    Measles,     Water    Berries,     and  415. 

Related  Vine  Troubles.  416. 

361.  Preliminary  Yield  Tables  for  Second- 

Growth   Redwood.  417. 

362.  Dust  and  the   Tractor  Engine. 

363.  The  Pruning  of  Citrus  Trees  in  Cali-  418. 

fornia. 

364.  Fungicidal    Dusts   for   the    Control    of  419. 

Bunt. 

366.  Turkish     Tobacco     Culture,     Curing,  420. 

and   Marketing. 

367.  Methods  of  Harvesting  and  Irrigation  421. 

in  Relation  to  Moldy  Walnuts.  422. 

368.  Bacterial      Decomposition     of      Olives 

During  Pickling.  423. 

369.  Comparison     of     Woods     for     Butter 

Boxes.  424. 

370.  Factors   Influencing   the    Development 

of  Internal  Browning  of  the  Yellow  425. 

Newton  Apple.  426. 

3  71.  The   Relative    Cost   of   Yarding    Small 

and   Large   Timber.  427. 

373.  Pear    Pollination. 

374.  A    Survey    of    Orchard    Practices    in  428. 

the     Citrus     Industry     of     Southern 
California. 

375.  Results   of   Rice   Experiments   at   Cor-  429. 

tena,   1923,  and  Progress  in  Experi-  430. 

ments  in  Water  Grass  Control  at  the  431. 

Biggs   Rice  Field   Station,    1922-23. 
377.   The  Cold  Storage  of  Pears.  432. 

380.   Growth    of    Eucalyptus    in    California 

Plantations.  433. 

382.   Pumping    for    Draininge    in    the    San 

Joaquin   Valley,    California.  434. 

385.  Pollination  of  the  Sweet  Cherry. 

386.  Pruning     Bearing     Deciduous     Fruit  435. 

Trees. 

3  87.   Fig    Smut. 

388.  The   Principles   and   Practice   of    Sun- 
Drying  Fruit. 


Berseem  or  Egyptian  Clover. 

Harvesting  and  Packing  Grapes  in 
California. 

Machines  for  Coating  Seed  Wheat 
with   Copper   Carbonate   Dust. 

Fruit  Juice  Concentrates. 

Crop   Sequences  at  Davis. 

I.  Cereal  Hay  Production  in  Cali- 
fornia. II.  Feeding  Trials  with 
Cereal  Hays. 

Bark  Diseases  of  Citrus  Trees  in  Cali- 
fornia. 

The  Mat  Bean,  Phaseolus  Aconitifo- 
lius. 

Manufacture  of  Roquefort  Type  Cheese 
from  Goat's  Milk. 

Orchard   Heating  in    California. 

The  Utilization  of  Surplus  Plums. 

The  Codling  Moth  in  Walnuts. 

The  Dehydration  of  Prunes. 

Citrus    Culture   in    Central    California. 

Stationary  Spray  Plants  in  California. 

Yield,  Stand,  and  Volume  Tables  for 
White  Fir  in  the  California  Pine 
Region. 

Alternaria  Rot  of  Lemons. 

The  Digestibility  of  Certain  Fruit  By- 
products as  Determined  for  Rumi- 
nants. Part  I.  Dried  Orange  Pulp 
and  Raisin  Pulp. 

Factors  Influencing  the  (Quality  of 
Fresh  Asparagus  after  it  is  Har- 
vested. 

Paradichlorobenzene  as  a  Soil  Fumi- 
gant. 

A  Study  of  the  Relative  Value  of  Cer- 
tain Root  Crops  and  Salmon  Oil  as 
Sources   of   Vitamin   A  for   Poultry. 

Planting  and  Thinning  Distances  for 
Deciduous  Fruit  Trees. 

The  Tractor  on  California  Farms. 

Culture  of  the  Oriental  Persimmon  in 
California. 

Poultry  Feeding:  Principles  and  Prac- 
tice. 

A  Study  of  Various  Rations  for  Fin- 
ishing Range  Calves    as  Baby  Beeves. 

Economic  Aspects  of  the  Cantaloupe 
Industry. 

Rice  and  Rice  By-Products  as  Feeds 
for  Fattening  Swine. 

Beef   Cattle  Feeding  Trials,    1921-24. 

Cost  of  Producing  Almonds  in  Cali- 
fornia :  a  Progress  Report. 

Apricots  (Series  on  California 
and  Prices). 

The  Relation  of  Rate  of  Maturity  to 
Egg  Production. 

Apple  Growing   in  California. 

Apple  Pollination  Studies  in 
fornia. 

The  Value  of  Orange  Pulp  for  Milk 
Production. 

The  Relation  of  Maturity  of 
fornia  Plums  to  Shipping 
Dessert  Quality. 

Economic  Status  of  the  Grape  Industry. 

Range  Grasses  of  California. 

Raisin  By-Products  and  Bean  Screen- 
ings as  Feeds  for  Fattening  Lambs. 

Some  Economic  Problems  Involved  in 
the  Pooling  of  Fruit. 

Power  Requirements  of  Electrically 
Driven    Manufacturing    Equipment. 

Investigations  on  the  Use  of  Fruits  in 
Ice  Cream  and  Ices. 

The  Problem  of  Securing  Closer 
Relationship  Between  Agricultural 
Development  and  Irrigation  Con- 
struction. 


Crops 


Cali- 


Cali- 
and 


bulletins- 
No. 

436,  I.   The   Kadota   Fig.      II.   Kadota   Fig 

Products. 

437.  Economic    Aspects    of    the    Dairy    In- 

dustry. 
43  8.   Grafting  Affinities  with  Special  Refer- 
ence to  Plums. 

439.  The  Digestibility  of  Certain  Fruit  By- 

products as  Determined  for  Rumi- 
nants. Part  II.  Dried  Pineapple 
Pulp,  Dried  Lemon  Pulp,  and  Dried 
Olive  Pulp. 

440.  The    Feeding    Value    of    Raisins    and 

Dairy  By-Products  for  Growing  and 
Fattening  Swine. 

441.  The  Electric  Brooder. 

442.  Laboratory  Tests  of  Orchard  Heaters. 

443.  Standardization    and    Improvement    of 

California   Butter. 

444.  Series  on  California  Crops  and  Prices: 

Beans. 


-(Continued) 
No. 
445. 


Economic  Aspects  of  the  Apple  In- 
dustry. 

The  Asparagus  Industry  in  California. 

The  Method  of  Determining  the  Clean 
Weights  of  Individual  Fleeces  of 
Wool. 

448.  Farmers'      Purchase      Agreement     for 

Deep    Well    Pumps. 

449.  Economic   Aspects  of  the  Watermelon 
Industry. 

Irrigation  Investigations  with  Field 
Crops  at  Davis,  and  at  Delhi,  Cali- 
fornia. 

Studies  Preliminary  to  the  Establish- 
ment of  a  Series  of  Fertilizer  Trials 
in  a  Bearing  Citrus  Grove. 

Economic  Aspects  of  the  Pear  In- 
dustry. 


446 
447 


450. 


451. 


452. 


No. 

87.   Alfalfa. 
117.   The    selection    and    Cost    of    a    Small 

Pumping  Plant. 
127.   House  Fumigation. 
129.   The  control  of  Citrus  Insects. 
136.   Melilotus    Indica    as    a    Green-Manure 

Crop  for  California. 
144.   Oidium    or    Powdery    Mildew    of    the 

Vine. 
157.   Control  of  Pear   Scab. 
164.    Small    Fruit    Culture    in    California. 
166.   The   County  Farm   Bureau. 
178.  The  Packing  of  Apples  in   California. 

202.  County    Organization    for    Rural    Fire 

Control. 

203.  Peat   as   a   Manure   Substitute. 
209.   The  Function  of  the  Farm  Bureau. 
212.   Salvaging   Rain-Damaged   Prunes. 
215.   Feeding   Dairy   Cows  in    California. 

230.  Testing   Milk,' Cream,    and   Skim   Milk 

for  Butterfat. 

231.  The   Home  Vineyard. 

232.  Harvesting    and    Handling    California 

Cherries    for    Eastern    Shipment. 
234.  Winter     Injury     to     Young     Walnut 
Trees  During  1921-1922. 

238.  The   Apricot  in   California. 

239.  Harvesting     and     Handling     Apricots 

and  Plums  for  Eastern  Shipment. 

240.  Harvesting    and    Handling    California 

Pears  for  Eastern   Shipment. 

241.  Harvesting    and    Handling    California 

Peaches  for  Eastern   Shipment. 

243.  Marmalade     Juice     and     Jelly     Juice 

from  Citrus  Fruits. 

244.  Central  Wire  Bracing  for  Fruit  Trees. 

245.  Vine   Pruning   Systems. 

248.  Some  Common   Errors  in   Vine   Prun- 

ing and  Their  Remedies. 

249.  Replacing  Missing  Vines. 

250.  Measurement   of ,  Irrigation   Water   on 

the  Farm. 

252.  Support   for   Vines. 

253.  Vineyard   Plans. 

255.   Leguminous    Plants    as    Organic    Fer- 
tilizers  in    California    Agriculture. 

257.  The   Small-Seeded   Horse  Bean    (Vicia 

faba   var.   minor). 

258.  Thinning   Deciduous   Fruits. 

259.  Pear  By-Products. 
261.   Sewing  Grain  Sacks. 


CIRCULARS 
No. 


265. 
266. 

267. 

269. 
270. 
273. 
276. 

277. 

278. 
279. 
281. 


282. 

284. 
286. 
287. 
288. 
289. 
290. 
292. 
293. 
294. 
296. 

298. 

300. 
301. 
302. 
304. 
305. 
307. 
308. 
309. 
310. 

311. 

312. 


Plant   Disease  and   Pest   Control. 

Analyzing  the  Citrus  Orchard  b> 
Means  of  Simple  Tree  Records. 

The  Tendency  of  Tractors  to  Rise  in 
Front;   Causes  and  Remedies.     ' 

An   Orchard   Brush   Burner. 

A  Farm  Septic  Tank. 

Saving  the   Gophered   Citrus  Tree. 

Home    Canning. 

Head,  Cane  and  Cordon  Pruning  of 
Vines. 

Olive  Pickling  in  Mediterranean 
Countries. 

The  Preparation  and  Refining  of 
Olive  Oil  in  Southern  Europe. 

The  Results  of  a  Survey  to  Deter- 
mine the  Cost  of  Producing  Beef  in 
California. 

Prevention  of  Insect  Attack  on  Stored 
Grain. 

The  Almond  in   California. 

Milk  Houses  for  California  Dairies. 

Potato   Production   in    California. 

Phylloxera  Resistant  Vineyards. 

Oak   Fungus   in    Orchard   Trees. 

The  Tangier  Pea. 

Alkali   Soils. 

The    Basis    of    Grape    Standardization. 

Propagation   of   Deciduous   Fruits. 

Control  of  the  California  Ground 
Squirrel. 

Possibilities  and  Limitations  of  Coop- 
erative Marketing. 

Coccidiosis  of  Chickens. 

Buckeye  Poisoning  of  the  Honey  Bee. 

The   Sugar  Beet  in  California. 

Drainage  on  the  Farm. 

Liming  the   Soil. 

American  Foulbrood   and   Its   Control. 

Cantaloupe    Production    in    California. 

Fruit   Tree   and   Orchard  Judging. 

The  Operation  of  the  Bacteriological 
Laboratory  for  Dairy  Plants. 

The  Improvement  of   Quality  in  Figs. 

Principles  Governing  the  Choice,  Op- 
eration and  Care  of  Small  Irrigation 
Pumping   Plants. 


The  publications  listed  above  may  be  had  by  addressing 

College  of  Agriculture, 

University  of  California, 

Berkeley,  California. 


14m-6,'28 


